A CRITICAL REVIEW OF
RESEARCH ON THE NITROGEN
NUTRITION OF DAIRY PASTURES
IN VICTORIA
Dr Richard J. Eckard
The Institute for Land and Food Resources
The University of Melbourne
AND
Agriculture Victoria, Ellinbank
Department of Natural Resources and Environment
October 1998
Review Report commission by the University of Melbourne and Agriculture Victoria,
Department of Natural Resources and Environment, as part of the statewide nitrogen research
project Best Management Practices for nitrogen in intensive pasture production systems, with
additional funding provided by an Australian Research Council grant.
ISBN 0 7311 4270 5
Table of Contents
AIM ........................................................................................................................................................................ 2
INTRODUCTION ................................................................................................................................................ 2
PREVIOUS REVIEWS ............................................................................................................................................. 3
CONTEXT ............................................................................................................................................................ 3
RATES OF N FERTILISER ...................................................................................................................................... 4
Maximum rates .............................................................................................................................................. 4
Minimum rates ............................................................................................................................................... 5
NITROGEN RECYCLING UNDER GRAZING.............................................................................................................. 7
APPARENT RECOVERY ......................................................................................................................................... 7
NITROGEN EFFICIENCY ........................................................................................................................................ 7
Data not included in existing reviews ............................................................................................................ 8
RESIDUAL RESPONSES ......................................................................................................................................... 8
SOURCES OF NITROGEN FERTILISER ................................................................................................................... 10
NUTRITIVE VALUE OF PASTURE ........................................................................................................................ 11
FACTORS AFFECTING N RESPONSES OF PASTURE ............................................................................. 13
FREQUENCY OF HARVEST .................................................................................................................................. 13
TIME OF APPLICATION OF N FERTILISER ............................................................................................................. 13
FREQUENCY OF N APPLICATION ........................................................................................................................ 13
HEIGHT OF PASTURE AT TIME OF APPLICATION .................................................................................................. 13
TEMPERATURE EFFECTS .................................................................................................................................... 14
SOIL MOISTURE ................................................................................................................................................. 15
COMPARATIVE RESPONSE BY DIFFERENT SPECIES.............................................................................................. 15
BOTANICAL COMPOSITION CHANGE ................................................................................................................... 16
INTERACTIONS WITH OTHER FERTILISERS .......................................................................................................... 16
SOIL ACIDITY ..................................................................................................................................................... 16
N2 FIXATION ..................................................................................................................................................... 17
NON-SYMBIOTIC................................................................................................................................................ 17
SYMBIOTIC N2 FIXATION ................................................................................................................................... 17
RESPONSES UNDER GRAZING .................................................................................................................... 19
NITROGEN FOR HAY AND SILAGE ....................................................................................................................... 20
ENVIRONMENTAL IMPACTS AND LOSSES OF NITROGEN ................................................................ 21
LOSSES ON N FROM LEGUME BASED PASTURES ................................................................................................. 21
NITROGEN REMOVAL......................................................................................................................................... 21
ORGANIC NITROGEN .......................................................................................................................................... 21
MINERALISATION .............................................................................................................................................. 22
ATMOSPHERIC DEPOSITION ............................................................................................................................... 22
AMMONIA VOLATILISATION .............................................................................................................................. 22
NITRATE LEACHING .......................................................................................................................................... 23
DENITRIFICATION /NITRIFICATION ..................................................................................................................... 23
SURFACE RUN-OFF ............................................................................................................................................ 24
OTHER LOSS PROCESSES.................................................................................................................................... 24
CONCLUSIONS ................................................................................................................................................. 25
ISSUES TO CONSIDER FOR ANY FUTURE RESEARCH ............................................................................................ 26
ISSUES REQUIRING FURTHER RESEARCH ............................................................................................. 27
REFERENCES ................................................................................................................................................... 28
Copyright © The University of Melbourne and the Department of Natural Resources and Environment.
This publication may be of assistance to you, but the University of Melbourne and the State of Victoria and its employees do not
guarantee that the publication is without flaw of any kind, or is wholly appropriate for your particular purposes and therefo re disclaims
all liability for any error, loss or other consequence which may arise from reliance on any information contained herein.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
1
A CRITICAL REVIEW OF RESEARCH ON THE NITROGEN NUTRITION OF
DAIRY PASTURES IN VICTORIA
Richard Eckard
ILFR, University of Melbourne, Agriculture Victoria, Ellinbank
AIM
This review was commissioned in July 1997 jointly by the Institute for Land and Food Resources of
the University of Melbourne and Agriculture Victoria, Natural Resources and Environment. The
main aim of this review was:
1. to critically review all N fertiliser research conducted in Victoria applicable to intensive dairy
grazing systems;
2. to identify applicable information for use in a decision support system,
3. to identify critical knowledge gaps and, therefore,
4. to make recommendations for further research.
This review is not intended to cover research on flood irrigated pasture in Northern Victoria, as a
separate review is currently being prepared to compliment the current document.
INTRODUCTION
The nitrogen (N) nutrition of dairy pastures in Victoria could be classed into 3 main categories, based
on current practice:
1. Pastures which rely exclusively on clover N2 fixation. These pastures are usually at stocking rates
of 2.5 cows/ha or less;
2. pastures in which the contribution of clover is ignored, with N fertiliser being applied after each
grazing while moisture allows growth (usually at stocking rate above 3.5 cows/ha), and
3. pastures where strategic N fertiliser applications are applied when additional forage is required,
usually only during periods when clover growth is limited (Simpson 1987, Eckard 1996a,
McKenzie 1997). Currently the third option is the most popular.
At the outset it must be emphasised that N fertiliser practices on Victorian dairy farms have changed
dramatically over the past 10 to 15 years. In reviewing the N inputs in Australian legume based
pastures, Ellington (1986) reported that fertiliser N played an insignificant role in these systems.
However, 11 years later Eckard et al. (1997) reported an exponential increase in N fertiliser sales to
pastoral farmers in Victoria (Figure 1). It is mainly dairy farmers who use N fertilisers. Ellington
(1986) also stated that N fertiliser use and grass clover pastures were incompatible. This view is now
increasingly being challenged by the concept of strategic N fertiliser use during the cooler months of
the year, when clover is not actively growing nor fixing N (Newman et al., 1962; Eckard 1996a;
McKenzie & Jacobs 1997). Certainly the data in Figure 1 are strong evidence of the adoption of the
concept of strategic N by the dairy farming community.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
2
Nitrogen fertiliser sold (Kt)
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
1980
1982
1984
1986 1988
Year
1990
1992
1994
1996
Figure 1. Trends in N fertiliser sales to pastoral farmers in Victoria. Data drawn from sale figures of
a single fertiliser company in Victoria (Eckard et al. 1997).
Previous reviews
Three reviews adequately summarise the majority of N research data applicable to dairy pastures in
south eastern Australia published prior to 1987. The review by McGowan (1987) summarised and
collated the results of over 600 N fertiliser experiments conducted in Victoria over a 50-year period.
The review by Ellington (1986) focused on research conducted on the N dynamics (inputs and losses)
in legume-based pastures, with most of the focus applicable to temperate pasture production in
Victoria. The third review (Simpson 1987) focused on N nutrition of pastoral systems in Australia
providing a general overview of current knowledge of the N dynamics of grass/clover pastures.
Heavy reliance has also been placed on the comprehensive and applicable information contained in
the book by Whitehead (1995), where generic reference was required.
Context
One of the main issues to bear in mind when comparing responses of pastures in south eastern
Australia and New Zealand, to responses reported for temperate pastures in other countries, is both
the N and yield contribution of clover. Where responses are reported from grass and clover pastures
there are many instances where N fertiliser decreased total yield, due to an indirect suppression effect
on clover yield. Alternately, the N responsiveness of a highly productive grass and clover pasture
may be limited at certain times due to a relatively high soil mineral N status due to clover N 2 fixation.
Frame (1994) and Whitehead (1995) discussed these trends in sufficient detail.
Another factor limiting comparison of data from Europe is that the concept of “strategic” N fertiliser
applications, as defined in category 3 above, is only really applicable where winter temperatures do
not result in total cessation of grass growth for extended periods. With the long cold winters in most
of the European countries and the USA, there is no clear period when low soil temperatures limit
clover growth and soil N availability, but when grass growth is still possible, thereby allowing a grass
response to N fertiliser. This will be discussed in more detail.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
3
NITROGEN FERTILISER AND AGRONOMIC PRODUCTION
Rates of N fertiliser
Maximum rates
The average response curve presented in the McGowan (1987) review showed a curvilinear response
to a single application of N fertiliser up to a maximum of 215 kg N/ha per application. These data,
being an average of 92 experiments, appear to contradict more recent studies (Mundy 1993; Eckard
1996a; Eckard et al., 1997; McKenzie 1997; Eckard & Franks 1998), as well as internationally (Ball
et al. 1978; Eckard 1994; Eckard et al. 1995; Frame 1994; Whitehead 1995), where responses to a
single N application usually inflect between 60 kg N/ha to 100 kg N/ha per application (see Figures 3
and 4a). The response function presented by McGowan (1987) is also not supported by the following
figure in same review, which shows the annual yield response being steep initially, inflecting just
below 200 kg N/ha/yr, but increasing slowly thereafter beyond 1000 kg N/ha/yr. Obviously the data
of McGowan (1987) represent an average from a wide range of climatic and edaphic conditions,
whereas the recent data apply specifically to highly productive pastures perennial ryegrass pasture,
were all possible limitations to N response have been removed.
It is accepted that, in certain cases, economic responses to higher rates of N may be possible, as
reported by Newman et al. (1962) and for spring silage (Jacobs et al. 1998). Although most of the
data in Figure 4a (Eckard & Frank 1998) show some evidence of diminishing returns at the maximum
rate (60 kg N/ha), at least one of the responses appeared capable of a linear response to a higher N
rate. Where N fertiliser is applied specifically to achieve a high spring silage yield, a higher rate of N
may be applicable if the regrowth period is longer than the usual grazing interval (Jacobs et al. 1998).
However, in most cases, approximately 90% of maximum yield will be achieved by the application of
60 kg N/ha for any single regrowth period. Additional pasture growth may be achieved by the
application of N fertiliser in excess of 100 kg N/ha in any single application, but with efficiency
declining and losses increasing exponentially beyond an optimum rate (Carran & Clough 1995) of
around 60 kg N/ha per application (Eckard & Franks 1998).
Reasons for the differences between data sets are not clear but may relate to the regrowth period for
many of the trials reviewed by McGowan (1987), where N fertiliser was typically applied in the
autumn and harvested in the following spring. This period of regrowth may have been sufficient for
the growth of treatments without N fertiliser to “catch up”. Alternatively, the study of Chapman et
al. (1982) only allowed a 25 day regrowth period for all responses through the winter. Clearly this
would have been harvesting the pasture before it had chance to fully express its response to N in
winter. Most studies base the harvest interval on either a set number of days throughout the year, or
on district average practice (“when the pasture us ready for grazing”). In order to evaluate the true
response to N, regrowth periods must be based on some physiological growth stage of the plant,
although this would mean that treatments may have to be evaluated on different days.
Another obvious explanation lies with the basal fertility, soil organic matter and species composition
of the pasture. In two of the studies reviewed by McGowan (1987) soil P levels were clearly limiting
any appreciable response to N, being 6.5 ppm Olsen P (Chapman & Pugh 1983) and 9.9 ppm Olsen P
(Chapman et al. 1983). The recent data of McKenzie (1997) (Figure 2) shows that soil fertility and
species composition (Table 1) have a marked effect on the N responsiveness of pasture. A limitation
in many of the studies reviewed by McGowan (1987) was the lack of such detail, making contextual
interpretation of the N responses difficult. The recording of such additional data is vital in any future
N response study.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
4
Table 1. Soil analysis and species composition of two trial sites presented in Figure 2 (McKenzie
1997)
Site
Olsen P
Skene
S (CPC
pH
Ryegrass
Clover Weed
Other
(mg/kg)
K
method)
(water)
(%)
(%)
(%)
grasses
(ppm)
mg/kg
(%)
Simpson
39
313
100
5.2
53
9
1
35
Demo Dairy
17
215
11
5.4
25
12
3
60
When reporting the response to N fertiliser it is important to clarify whether the rate quoted is the
point of maximum yield or the estimated optimum application rate, based on criterion like 90% of
maximum yield or where the slope of the response equals 10. This may also explain some of the
difference between the McGowan (1987) review and current recommendations.
Minimum rates
Recent research in north west Tasmania (Eckard 1996a, 1996b) reported limited and highly variable
responses to N fertiliser rates below 15 kg N/ha per application (Figure 3). Chapman et al. (1982) and
Chapman et al. (1983) showed no response to N fertiliser when applied as 100 kg N split in 6
applications i.e. 16.7 kg N/ha/application. It would appear that the addition of small rates of N
fertiliser, while efficient in a pure grass pasture, being low in available mineral N (Eckard 1995;
Whitehead 1995), produce less predictable responses in a grass and clover pasture due to the
unknown temporal contribution of the clover. This does not mean that a marked response is not
possible, if clover growth and N2 fixation has been restricted for some reason.
1200
Dry Matter Yield (kg/ha)
1100
1000
900
800
700
600
Simpson
500
Demo Dairy
400
0
15
30
45
60
Nitrogen Fertiliser rate (kg N/ha)
Figure 2. The combined effect of species composition and basal fertility of the
N responsiveness of permanent pasture in Western Victoria (refer to Table 1).
Yields were estimated at 3-leaf emergence 46 days (mid-June 1996) after N
application (after McKenzie 1997).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
5
22
Growth rate (kg DM/day)
35
14
30
11
Elliott
Harcus
9
25
Henrietta
5
9
20
15
10
5
0
0
10
20
30
40
50
Nitrogen Fertiliser Rate (kg N/ha)
60
70
Figure 3. A typical pattern of response of a perennial ryegrass/clover pasture to N fertiliser.
Nitrogen fertiliser was applied mid-June on a warm coastal site (Harcus), a cold site
(Henrietta) and an intermediate site (Elliott Research Station), and harvested at 3-leaf stage
62, 98 and 82 days later respectively. The figures on the graph indicate the efficiency of N
fertiliser in terms of kg DM/kg N (Eckard 1996).
3500
11.7
14.3
3000
LSD 5% - 1995 N rate
14.0
9.8
11.2
9.3
2500
Dry Matter Yield (kg DM/ha)
LSD 5% - 1995 Pre : Post
15.9
11.3
12.8
9.1
LSD 5% - 1996 N rate
Pre95
Post95
10.9
2000
LSD 5% - 1996 Pre : Post
Pre96
12.7
Post96
9.7
1500
9.0
10.6
14.5
1000
500
0
0
10
20
30
40
50
60
Nitrogen Fertiliser Rate (kg N/ha)
Figure 4a. The DM yield response of a perennial ryegrass and white clover pasture to increasing
rates of N fertiliser application. Data are presented as averages of five pre mid winter (1 st July)
responses in 1995 (Pre95; --×--) and 1996 (Pre96; ×) and five post mid-winter responses in
1995 (Post95; - -  - -) and 1996 (Post96; ). Numeric values indicate the N-use efficiency in
terms of kg DM per kg N applied. Vertical bars indicate l.s.d (P =0.05) (Eckard & Franks 1998).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
6
Nitrogen recycling under grazing
Nitrogen cycling in a grazed pasture system is not a highly efficient process, as the spatial variability
in redistribution of nutrients through dung and urine is antagonistic to efficient recycling. Dairy cattle
excrete 75 to 80% of the N they consume (Whitehead 1995), with only 30 to 40% of recycled N
ultimately being available for plant growth (Ball 1979; Ball & Ryden 1984; Whitehead 1995).
Cattle dung is reasonably constant in N content relative to DM consumed, being approximately 0.8 g
N/100 g consumed (Whitehead 1995). Consequently, changes in dietary N concentration are reflected
more in the proportion of dietary N excreted in the urine, being around 45% on a diet of 1.5% N to
around 80% on a diet of 4% N (Whitehead 1995). Cattle urine contains between 2 to 20g N/l, with
around 8 to 12 urination events per day, at a volume of around 1.5 to 3.5l, covering an area of
approximately 0.4 to 0.8 m2 per urination (Simpson 1987; Whitehead 1995). The ranges quoted above
relate mainly to the protein and moisture content of the diet. Urine patches may therefore receive
between 75 and 875 kg N/ha effective rate. Other figures quoted range between 670 and 1110 kg
N/ha (Whitehead 1995), although an effective rate of >1600 kg N/ha can be calculated from the
ranges given above.
Faecal N is largely organic in form and requires mineralisation by soil micro-organisms before
becoming available to plants (Simpson 1987). Cattle dung contains between 1.5 to 4% N (DM basis),
with around 7 to 15 defecation events per day, at a relatively constant mass of around 0.3 kg, a dry
matter content of 8 to 16 % and covering an area of approximately 0.7 m2 per defecation (Simpson
1987; Whitehead 1995). Dung patches may therefore receive between 800 and 1070 kg N/ha effective
rate. Other figures quoted by Whitehead (1995) give a range of between 750 and 1330 kg N/ha
effective rate.
On an annual basis the above N inputs may account for around 160 to 240 kg N/ha/yr from urine and
around 50 to 80 kg N/ha from dung (Whitehead 1995).
Apparent recovery
The apparent recovery of N fertiliser applied to a pasture, has been reported between 50 and 80%,
and often around 65 to 70% (Whitehead 1995). However, lower apparent recovery rates (below 50%)
have been reported from older studies conducted in Australia and New Zealand, with losses through
denitrification and volatilisation being implicated (see Simpson 1987). Whitehead (1995) points out
that, assuming a partitioning of N in the ratio of 2:1 herbage to stubble and root, a recovery of 67%
would represent complete uptake of N. Care should therefore be exercised in the interpretation of N
recovery studies to ensure that the N recovery quoted accounts for N in the stubble and root
component.
Nitrogen efficiency
Averaging all the applicable trials in Victoria, McGowan (1987) reported an average DM yield
response of 10 kg DM/kg N applied. This average is probably a reasonable estimate given the
following:
1. The average response is drawn from trials conducted on a range of pasture species composition
from undefined composition (i.e. winter pastures), Kikuyu, millet, oats, through to the highly
nitrophylous perennial and annual ryegrasses. The efficiencies of N utilisation ranged from
negative values through to 47 kg N/ha/application (Morgan & Rayner 1941; Chapman &
McGowan 1980). In trials where the N responsiveness of pasture species were compared annual
and short rotation ryegrasses averaged 21 kg DM/kg N and perennial ryegrass averaged 13.2 kg
DM/kg N applied.
2. The data were averaged over regrowth periods ranging between 2 and 16 weeks. In some of the
studies referenced herbage responses were evaluated after regrowth periods in excess of 120 days
(Colwell 1977); these data were not included in the averages as the unfertilised pasture would
have had sufficient time to catch up with the N fertilised pasture thereby nullifying any response.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
7
3. The responsiveness of a pasture to N fertiliser is directly proportional to the potential pasture
growth rate. As these data were averaged over a range of months of application, the potential
response and, therefore, efficiency varied greatly. When these averages were split according to
the month of application the average efficiencies ranged from 8.8 kg DM/kg N in May through to
a high of 13.5 kg DM/kg N in September.
4. In many cases the basal soil fertility of the pastures in the trial are not specified. However, some
indication of this range may be elucidated from the study by Norman (1962) where annual P
applications were 22 kg P/ha/yr, and the study by Chapman and Pugh (1983) and Chapman et al.
(1983) where annual P applications were 100 kg P/ha at 6.5 ppm and 9.9 ppm Olsen P
respectively. Clearly responses to N fertiliser would be limited at these lower Olsen P levels.
5. When a more select sub-set of trials were averaged, excluding some of the obvious issues raised
above, the average response reported was 11.3 kg N/ha. This average included seasonal
differences and some species differences.
Data not included in existing reviews
In 1965 Wolfe and Crofts (1969) reported the N response efficiency of a pure Kangaroo valley
perennial ryegrass as 3.3, 12.3, 8 and 25.1 kg DM/kg N, with 42 kg N/ha applied 6 weekly, and 10.6,
16.5, 10.6 and 23.5 kg DM/kg N, with 70 kg N/ha applied 6 weekly, for early autumn, late autumn,
early winter and late winter respectively. These responses were reported to be “about five times that
of a New Zealand ryegrass/white clover pasture investigated at the same location” (Wolfe & Crofts
1969). Crofts (1965) reported consistent responses of 15 to 23 kg DM/kg N applied to a perennial
ryegrass /clover pasture south west of Sydney.
Recent data on pure perennial ryegrass/clover pastures, with higher levels of soil fertility (25 - 40
mg/kg Olsen P) have shown that these values can be greatly improved (Eckard 1996a; McKenzie &
Jacobs 1997). The data of Eckard (1996a) in North West Tasmania (Figure 3) reported mid-winter
(June) N responses of 22 kg DM/kg N, 14 kg DM/kg N and 9 kg DM/kg N on a warm coastal site, an
upland site and a cold back-country site respectively. Likewise, McKenzie & Jacobs (1997) showed a
range of responses from 2 kg DM/kg N through to 23 kg DM/kg N (average efficiency of 6 to 16 kg
DM/kg N) (Figures 5a & 5b), explaining the differences in terms of species composition, basal
fertility and temperature regime (see Figure 3). Certainly we now have farmers recording responses
in excess of 30:1 on pure short-rotation ryegrass pasture, with responses rarely below 10:1 even at the
coldest time of the year (Versteden 1997).
One of the limitations of the nitrogen efficiency term, expressed as kg DM/kg N applied, is that it has
no time context. It has been suggested that N efficiency be expressed as growth rate increase per kg N
applied (R. Simpson pers comm., 1998, CSIRO, Plant Industries, Canberra). The data presented in
Figure 4a show pre mid-winter DM yields to be lower than post mind-winter yields (a function of
growth rate potential), however, N efficiencies (kg DM/kg N) appeared to be of similar magnitude.
What these data do not reflect is the time taken to achieve the responses i.e. pre mind-winter
responses took on average 87 days to reach target yields, whereas post mind-winter responses took 57
days. In order to accommodate the time context the data of Figure 4a are presented in Figure 4b in
terms of growth rate increase per kg N applied (kg DM/d/kg N). From the data in Figure 4b it
becomes clear that post mid-winter responses are most efficient between the 30 and 45 kg N/ha rate,
with lower efficiencies recorded at the 15 and 60 kg rates. In this case only the 60 kg N/ha rate is
clearly less efficient, when the range (individual years) of lower rates in taken into account. In the pre
mid-winter period the lighter rate of N appears most consistently efficient, perhaps due to a high
residual N in the soil from the summer period. However, in the pre mid-winter period there appears to
be no clear detriment to the application of 60 kg N/ha, and the range in efficiency at the 45 kg N/ha
rate is not clearly lower than that at 15 kg N/ha.
Residual responses
Residual responses refer to the effect of an N fertiliser application in the second regrowth period post
application, or ‘carry-over’ N. Residual response data reviewed by McGowan (1987) were highly
variable. The trend that emerged was that at lower N application rate residual effects may even be
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
8
negative; perhaps due to the stimulation of the plant to a higher growth rate initially, but with
insufficient fertiliser N to maintain the higher growth rate through to a maximum. At the
recommended optimum (50 kg N/ha) residual effects were largely neutral, perhaps indicating that N
was efficiently utilised. At higher rates of N application positive residual effects were noted,
indicating potential inefficiency in utilisation for the initial regrowth.
Recent research in Western Victoria (McKenzie 1997) demonstrated residual responses in the second
regrowth post N application. While average response efficiency to the first harvest post N application
was 16, 13, 11 and 9 kg DM/kg N, residual responses, relative to an unfertilised control treatment,
ranged between 7, 6, 5 and 6 kg DM/kg N, for N rates of 15, 30, 45 and 60 kg N/ha.
Growth rate increase/kg N
0.3
0.25
kg DM/d/kg N
0.2
1995 pre
1995 post
1996 pre
1996 post
Avg Pre
Avg Post
0.15
0.1
0.05
0
0
10
20
30
40
50
60
Nitrogen Fertiliser Rate (kg N/ha)
Figure 4b. Nitrogen fertiliser use efficiency, in terms of kg DM/d/kg N, response of a perennial
ryegrass/clover pasture in north west Tasmania. Responses are presented as pre and post midwinter (1st July) averages (after Eckard 1998).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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Figure 5a. Dry matter yield responses to increasing rate of N fertiliser at Demo Dairy,
Terang, Western Victoria. Yields were estimated at 47 (Mid-April), 52 (early-May), 42
(mid-May), 38 (early-June) and 46 (mid-June) days after N application for the dates listed
(McKenzie 1997).
Figure 5.b Dry matter yield responses to increasing rate of N fertiliser at Simpson, Western
Victoria. Yields were estimated at 36 (Mid-April), 36 (early-May), 28 (mid-May), 24 (earlyJune) and 30 (mid-June) after N application for the dates listed (McKenzie 1997).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
10
Sources of nitrogen fertiliser
There are some individual studies internationally that have documented one source of N as superior
to another, however, most studies have shown little difference between sources (Whitehead 1995).
Grass roots can absorb and effectively utilise ammonium or nitrate to produce similar yields;
consequently, since most forms of N are rapidly transformed into nitrate in the soil, differences
between sources are seldom detectable in grassland systems (Simpson 1987). Whitehead (1995)
concludes that the specific conditions at the time of application will determine which source will
either volatilise or denitrify more, and that these conditions may vary from day to day. For example,
in a cold waterlogged soil an ammonia source may be more effective than a nitrate source.
Conversely, when the soil is drier and warmer a nitrate-N source is usually more effective than
ammonia-N sources.
McGowan (1987) reviewed 66 trials involving comparisons between sources of N fertiliser. Although
Urea was marginally less efficient than ammonium nitrate, calcium ammonium nitrate and
ammonium sulphate, it remains that most cost efficient fertiliser when the relative efficiencies are
accounted for. Urea was also viewed (McGowan 1987) as a superior source of N fertiliser in wet
conditions where excessive denitrification could occur. As most of the N applied to dairy pastures is
during the wetter months of the year, urea remains the source of choice. However, there may also be a
future cost associated with the choice of N source, as some acidify the soil more rapidly than others
(see section on soil acidity).
In 1996 a number of collaborative N response studies were conducted in Tasmania (Eckard 1996a,
1996b; Hopson 1996) and Western Victoria (McKenzie & Jacobs 1997). In north east Tasmania,
Eckard (1996b) reported autumn applications of potassium nitrate to yield significantly (P>0.05)
more pasture than urea, with ammonium nitrate treatments non-significantly different from either.
However, when the same treatments were applied to different plots in the spring a declining trend in
yield response was reported, with ammonium nitrate yielding the highest, followed by potassium
nitrate, with urea being significantly (P<0.05) lower yielding (12%) than ammonium nitrate. In
contrast to the above, similar comparisons in south west Tasmania (Hopson 1996) found no
significant difference between urea, ammonium nitrate and potassium nitrate applied in four
sequential top-dressing through the winter. In Western Victoria McKenzie (1997) and McKenzie &
Jacobs (1997) concluded that, apart from a superior response to DAP where soil fertility was
marginal, there was no yield advantage between sources of N on three separate sites.
One concern with the data in the McGowan (1987) review is that the form of urea fertiliser changed
around the early 1980’s from prilled sources to a harder granular source coated with a chemical resin.
This process was designed to reduce volatilisation losses and overcome the deliquescence
(Whitehead 1995). With all the studies reviewed being prior to the early 1980’s one would expect
urea to have been less efficient than the current coated granular product. Perhaps this is an
explanation for some of the differences between older studies and those conducted in recent years.
Nutritive Value of Pasture
In a pure grass pasture system the application of N fertiliser results in an increase in plant N content
mimicking the shape of a diminishing yield response curve (Eckard 1994, Whitehead, 1995). Apart
from season and physiological variation, the N content of the grass component of a grass/ clover
pasture depends mainly on the clover content and the efficiency of clover N 2-fixation. Trials
investigating the effect of N fertiliser on the N content of a grass/ clover pasture may be subject to
confounding from residual clover N sources. These points are borne out in the data reviewed by
McGowan (1987) showing highly variable results. McGowan (1987) reported a dearth of data on the
nutritive value of pasture from N response trials in Victoria.
In more recent studies in Western Victoria (Simpson and Demo Dairy, Terang) McKenzie (1997)
reported increasing rates of N increased CP for all N application times. Crude protein ranged from 13
(no N) to 23 % (60 kg N/ha) at DemoDairy and from 21 (no N) to 27 % (60 kg N/ha) at Simpson.
Pasture DMD increased with increasing rates of N from 61 (no N) to 73 % (45 kg N/ha) at
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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DemoDairy, but remained unchanged at Simpson (better species composition). Increasing rates of N
increased ME for all application times over a range of 8.3 (no fertiliser) to 10.4 MJ/kg DM (45 kg
N/ha) at DemoDairy. Pasture ME was unaffected by treatment at Simpson. The water soluble
carbohydrate (WSC) content of the pasture was unaffected by increasing rates of applied N at
DemoDairy, but was decreased by increasing rates of N at Simpson over a range of 9.3 (no fertiliser)
to 3.1% (45 kg N/ha ). These data probably reflect higher growth rate responses to N at the Simpson
site, relative to the Demo Dairy site. Increasing rates of N decreased NDF for all application times
over a range of 61 (no fertiliser) to 51 % (45 kg N/ha) at DemoDairy. At Simpson, however, NDF
was unaffected by treatment. Similar trends to the above were recently reported from adjacent
experiments investigating the effect of N fertiliser on silage quality (Jacobs et al. 1998).
Rogers (1985) reported a reduced milk protein and solids-not-fat in the milk of cows on high N
fertilised pastures relative to lower N pastures. These differences were attributed to a higher N and
lower soluble carbohydrate content of the herbage.
Although nitrate toxicity has been associated with N fertilised annual ryegrass (and short rotation
ryegrass) (Eckard 1990b), perennial ryegrass is not considered to accumulate toxic quantities of
nitrate (Crawford et al.1961; Wright & Davison 1964; Darwinkel 1975; Eckard & Dugmore 1994).
This is borne out by the findings of McKenzie (1997) who reported herbage nitrate N (N03--N)
content to be unaffected by N treatment in two experiments and at three sites.
McKenzie (1997) recently investigated the effect of N fertiliser on pasture mineral composition,
although significance was not attached to the data reported. Major minerals influenced by treatment
during autumn included P, K, S, Mg, Na, Ca and Cl. Nitrogen fertiliser elevated the herbage content
of P, K, S, Mg, Na and Cl, while herbage Ca was depressed by the addition of N. Trace minerals
increased in herbage by the addition of N were Zn and Cu, while herbage Mn content was depressed
by the addition of N.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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FACTORS AFFECTING N RESPONSES OF PASTURE
Frequency of harvest
Based on the data reviewed by McGowan (1987) an 8-week harvesting interval is superior in yield
and efficiency to a 4 week interval. Treatments harvested at 8-weekly intervals had higher initial
growth rates, growing up to 50% faster than those harvested 4-weekly. However, over a 16 week
period total efficiency and yield from a single N application was 22 kg DM/kg N versus 19 kg DM/kg
N at the 4 and 8-weekly harvest respectively. McGowan (1987) makes reference, on a number of
occasions, to the suppression of yield in the second harvest subsequent to N application, a view
supported by some anecdotal information. However, these anecdotal observations could be affected
by the perception of the relative difference between the first response, being accelerated by N
fertiliser, and the regrowth without N fertiliser. These findings are not consistent with trends from
other countries (Whitehead 1995) and therefore require further explanation or investigation.
Time of application of N fertiliser
The strategic application of one or two topdressings of N fertiliser has long been advocated as an
economical practice in intensive dairy production systems of south eastern Australia (Newman et al.
1962; Simpson 1987; Eckard 1996a; McKenzie & Jacobs 1997). If followed by appropriate grazing
management and utilisation, a single application of 50 to 60 kg N/ha in late winter or early spring,
does not affect the later growth of clover, which can be relied on as the N source for the remainder of
the growing season (Simpson 1987).
One of the issues identified by Simpson (1987) is that the period of greatest N demand by the pasture
does not always coincide with the period of greatest N supply via legume N 2 fixation or
mineralisation. As a result the N supply in grassland fluctuates through the season, being deficient
during peak growth periods. Likewise, soil nitrate accumulates during the summer and autumn when
rainfall is too low to support pasture growth, but sufficient moisture remains in the soil for
mineralisation of organic soil N (Simpson 1962).
Collating the data from 68 trials, involving comparisons between times of application, McGowan
(1987) showed peak responses to N to be during the peak growth months (September, 13.5 kg DM/kg
N; October, 12.6 and November, 12.3 kg DM/kg N). Responses over the winter months averaged 10.8
kg DM/kg N, with responses during the dry summer predictably lower (6.7 kg DM/kg N).
Delaying the application of N fertiliser to a ryegrass pasture, from immediately after grazing by 8 and
15 days reduced potential yield responses by 8 and 15% on a 27 day rotation (Mundy and Wilson
1995b). Pastures were noted to be most responsive to N when applied within 7-8 days of initial
defoliation. The application of N fertiliser 4 days prior to defoliation appeared to have little effect on
DM yield responses. This could be due to N fertiliser requiring about 4 to 5 days to dissolve and
reach the root zone in available form (Whitehead 1995).
Frequency of N application
Data collated from two studies (McGowan 1987), comparing frequencies of N application showed
more frequent N applications to be marginally more efficient (12.9 kg DM/kg N) than less often (11.6
kg DM/kg N). Certainly these data are in agreement with trends shown by Eckard et al. (1995), where
smaller N applications more frequently (4-weekly) were shown to be more efficient than larger
applications of N with lower frequency (6- and 8-weekly). In principle, the ideal nutrition for a crop
would involve a constant supply of N at a low rate. However, earlier discussion showed that, while
this may be true in a pure grass pasture, clover may confound responses to small applications of N
fertiliser in a grass/clover system.
The effect of rotation length on the overall response to N, on a paspalum dominant pasture in
Northern Victoria, after 60 days was 13.8, 16.3, 18.8 and 23.8 kg DM/kg N for 10, 15, 20 and 30 day
rotations (Mundy and Wilson 1995a).
Height of pasture at time of application
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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McGowan (1987) reviewed six trials in which the height of pasture at time of N fertilisation was
compared. Although the data were highly variable a general trend of lower efficiency of N use was
noted where pastures were both too short (1.2 to 2.5 cm; 13.8 kg DM/kg N) and too long (7.5 to 8.9
cm; 16.5 kg DM/kg N), relative to 5 cm pasture height (18 kg DM/kg N). If the pasture is grazed too
short then root reserves will require some repletion, before top growth can be maximised.
Conversely, a long pasture may have already reached its peak growth rate and, although this can still
be boosted further by the addition of N fertiliser, the rate of N uptake is slower.
Temperature effects
In winter, soil temperatures are often too low for any appreciable mineralisation or N 2 fixation.
Coupled with low plant growth rate and high rainfall (leaching), this results in early spring growth
being severely N deficient (Simpson 1987). Although clover is capable of limited growth at soil
temperatures of >3°C, N2 fixation, mineralisation and clover growth are severely restricted at soil
temperatures below 9 - 10°C (ADAS 1982, Frame & Newbould 1986; Hart 1987; Leconte 1987;
Kemp & Guobin, 1992; Whitehead 1995). Temperate grasses, like perennial ryegrass on the other
hand, will continue to grow at temperatures above 4 to 5°C (Blackman 1936; MacDuff & Dhanoa
1990; Kemp & Guobin, 1992; Frame 1994; Eckard 1994; Whitehead 1995). This means that, most of
the lower-altitude grass/ clover pastures of south eastern Australia could be severely N deficient
between May and August. This principle was illustrated by Eckard (1994; 1996a) in Figure 6.
Growth rate (kg DM/ha/day)
Eckard (1996a) compared the N responsiveness of pastures at three locations (temperature regimes)
of similar pasture composition and fertility in North West Tasmania (Figures 3 & 4a), reporting soil
temperature to play an important role in determining pasture growth rate, and thereby N
responsiveness. The N responses measured at the three locations were 22, 14 and 9 kg DM/kg N, with
average soil temperatures for the 14 days post N application being 9.0, 6.5 and 4.9°C. However
Eckard & Franks (1998) concluded that it was difficult to isolate the effect of soil temperature on N
responsiveness from other environmental factors. Their study highlighted the overall effect of
seasonal change from autumn into mid-winter as distinct from the responses post mid-winter.
Whitehead (1995) and Frame (1994) list light intensity, temperature, daylength and water supply, as
well as soil factors, as determinants of grass growth rate and thus potential N fertiliser response.
60
Grass N
demand
Clover potential
50
40
30
Autumn
Break
20
10
?
?
0
Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
Jul
Figure 6. An illustration of the relationship between grass growth and the ability of clover to
supplement the N nutrition of the pasture. The arrows indicate periods of the year when the N
demand of the grass exceeds the clover's ability to supply (Eckard 1996a).
Based on research in western Victoria McKenzie and Jacobs (1997) showed responses of 12 – 17 kg
DM /kg N in autumn (12 °C average soil temperature), 7 - 15 kg DM/kg N in winter (8 °C average
soil temperature) and 23 kg DM/kg N in spring (11 °C average soil temperature) from pasture of
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998) 14
reasonable fertility and species composition. Likewise, the responses from a pasture of poor species
composition and lower basal fertility was in the range of 7 – 8 kg DM/kg N, 6 – 7 kg DM/kg N and
17 kg DM/kg N for the same seasons as above.
The N responsiveness of pasture is clearly affected by soil temperature. However, Eckard & Franks
(1998) concluded that the seasonal direction of temperature change is an important factor required to
keep an individual soil temperature in context. These data sets should be the subject of a simple
modelling exercise to investigate this further.
Soil Moisture
The effect of soil moisture on the N responsiveness of pasture, apart from the dissolving or leaching
influence on N fertiliser, is largely through the indirect effect of soil moisture on pasture growth
potential. If soil moisture is limiting pasture growth then the N responsiveness of the pasture will
likewise be affected (Whitehead 1995).
The data of Mundy and Wilson (1995c) showed losses of 21% of N applied to pasture with a dry soil
surface (Evaporation – Rainfall =50) reducing to 9% with 10mm rainfall and 6% with 50mm rainfall.
Similarly, losses of 2% were recorded where N was applied to pasture with moist soils (E-R=0).
Apart for the data presented above, the direct effect of soil moisture on the N responsiveness of
pasture has not been investigated in south eastern Australia, particularly the effect of water-logged
soils. However, these effects could easily be modelled through a model of pasture growth. For most
of the season in which strategic N applications are recommended soil moisture is not a limiting factor
to pasture growth However, there is an increasing number of farmers who would apply N fertiliser
after the occasional summer storm if short of pasture. Unfortunately, little data are available to
provide recommendations on summer rainfall intensity versus N responsiveness of pasture. This too
could be the subject of a simple modelling exercise.
Comparative response by different species
Although limited data were available directly comparing the N response efficiency of different
pasture species, sufficient data were available in the review by McGowan (1987) to show annual
ryegrass (19.1 kg DM/kg N) to be on average 20% more N efficient than perennial ryegrass (15.2 kg
DM/kg N). In Tasmania, Hopson (1996) recently reported the efficiency of four sequential
applications of N fertiliser on two pasture types through the winter. A 60% perennial ryegrass and
32% white clover pasture yielded 7.1 kg DM/kg N, while a 30% perennial ryegrass, 26% tall fescue,
20% cocksfoot and 18% white clover pasture yielded 5.1 kg DM/kg N. Similar differences were
reported from recent trials in Western Victoria (McKenzie & Jacobs 1997) comparing ‘good’ (13.7
kg DM/kg N) and ‘poor’ (8.2 kg DM/kg N) pasture species composition (pure ryegrass vs low
ryegrass content). These data are in agreement with other studies internationally comparing tall
fescue (8.7 kg DM/kg N), perennial ryegrass (14.1 kg DM/kg N) and annual ryegrass (25.2 kg DM/kg
N) under grazing Eckard (1994), a trend supported by the review of Whitehead (1995).
In 1964 and 1965 Wolfe & Crofts (1969) studied the comparative N responses of a range of pure
grass pasture species, including cultivars of cocksfoot (Brignoles, Currie, Danish), perennial ryegrass
(Kangaroo valley, Victorian), tall fescue (Demeter, Oregon) and Phalaris aquatica, in southern New
South Wales. While no significant differences were noted between the N responses of different
species in spring, perennial ryegrass and phalaris N responses were significantly lower in summer,
kangaroo valley ryegrass N responses were significantly higher than most in winter and both tall
fescues higher in autumn. These trends reflect the different species response to temperature, and thus
difference seasonal growth potential.
One concern with some of the studies reviewed by McGowan (1987) was that in a number of cases
the pasture in the trials was not characterised, as these pastures could have included a range of less
productive species. These concerns are supported by the recent data of McKenzie (1997) discussed
above.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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Botanical composition change
McGowan (1987) reviewed 8 trials in which pasture species composition change was monitored,
showing an average decline in clover content of 60% following a single application of N fertiliser.
However, the range in data (19 – 93 %) gives some indication of the variability between the trials,
indicating that other management factors during the regrowth period can play a significant role in
influencing species composition. This is borne out by further data reviewed by McGowan (1987)
where 73 out of 83 trials showed significant depressions in clover content. However, the N was
applied in autumn with all 8 sites being left until mid-spring before next harvest; this period of
regrowth is almost certain to result in clover suppression, even in the absence of N fertiliser. A
number of early authors appear to have agreed that a single application of N fertiliser in late winter
will not cause severe grass dominance or impair clover growth, provided the additional forage
produced is consumed by mid-spring (Newman et al. 1962; Croft 1965; Ball et al. 1978). The data of
Rogers (1985) appear to substantiate this where increasing the stocking rate appeared to reduce the
negative effect of N fertiliser on clover content.
It is now generally accepted that N fertiliser applications do not suppress clover growth per se, but
result in a stimulation of the more nitrophylous grass at the expense of the less aggressive companion
legume (Whitehead 1995). One would therefore ask the question whether the data from the 83 trials
would have shown the same trends if the N responses were defoliated in the timeframe commonly
applied on intensively grazed dairy pastures.
One common failing in N fertiliser response research is that most trials compare rates of N fertiliser
under similar defoliation regimes. As N fertiliser accelerates plant growth this would result in the
treatments being evaluated at different physiological stages of regrowth. Likewise this differential
growth rate would mean that clover in the higher N treatments would be subject to shading for longer
than in the lower N treatments, leading to perhaps unfair comparisons of the effect of N fertiliser on
clover composition.
Interactions with other fertilisers
Logically any factor limiting grass growth potential will limit the potential response to N fertiliser.
Whitehead (1995) reviewed ample evidence demonstrating that where low soil P, K, S and soil pH
are limiting grass growth potential, the response to N fertiliser would also be limited. However, in
averaging the data from all trials investigating interactions between N and other nutrients in Victoria,
McGowan (1987) could find little evidence to support this. There were, however, specific studies
reviewed by McGowan (1987) that reported significant interactions between applications of N and P
and K. In one trial, autumn responses to N were 8.3 kg DM/kg N, compared to 18.3 kg DM/kg N with
both N and P+K (Chapman & Pugh 1983; McGowan 1987). Spring responses were not as dramatic,
being 19.9 kg DM/kg N with N compared to 24.7 kg DM/kg N with N, P & K. A major problem
noted in the data of Chapman & Pugh (1983) and Chapman et al. (1982) was that soil P levels were
too low to expect a significant response to N (Olsen P of 6.5 and 9.9). The data from some of the
other experiments appears confounded with a strong clover response to P applications, resulting in
clover dominance, high rates of N fixation and consequently limited response to N fertiliser.
Although confounded with poor species composition, the recent data of McKenzie (1997) from
Western Victoria indicate a pasture with lower basal fertility to be less N efficient than pastures
adequate in basal nutrient. This principle is important to keep in mind when making
recommendations to farmers.
Soil acidity
Most of the data reviewed by McGowan (1987) showed little or no short-term effect of occasional
applications of N fertiliser on soil pH. However, this may merely reflect a high soil buffering
capacity. In two studies ammonium sulphate was shown to acidify the soil more rapidly than other
sources (McGowan 1987), confirming data reported elsewhere (Eckard 1986; Eckard 1990a;
Whitehead 1995).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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N2 Fixation
Non-symbiotic
Although non-symbiotic N2 fixation appears to be an important source of N in extensive grasslands,
the relative contribution in intensively managed pastures appears limited (Ellington 1986),
particularly where N fertiliser has been used (Whitehead 1995). However, Ellington (1986) indicated
that data from Australian conditions was limited to inferences only, with estimates of 10 to 16 kg
N/ha/yr from New Zealand data (Ball 1979; Simpson 1987). Further research on the N 2 fixation
contribution of free living organisms to the soil N pool would be a useful addition to the knowledge
base on the cycling of N in pastoral systems, particularly the potential for the inoculation of grass
seed with bacterium like Azospirillum spp (Whitehead 1995). Estimates from studies in other
countries range between 24 and 49 kg N/ha/yr (see review of Ellington 1986), while those from more
intensive pasture systems seldom exceed 5 to 8 kg N/ha/yr, being largely undetectable where N
fertiliser was used in the previous six weeks (Whitehead 1995). Non-symbiotic N2 fixation is unlikely
make a significant contribution to N nutrition of intensive pasture. A more detailed analysis of nonsymbiotic N2 fixation can be found in Whitehead (1995).
Symbiotic N2 fixation
Symbiotic N2 fixation is a major source of N for both extensive and intensive pastoral systems in
south eastern Australia. The literature available on the N2 fixation of legumes in temperate Australia
is voluminous and mainly beyond the scope of this review. What is most pertinent to the current
review is the effect of N fertiliser on legume N2 fixation, as well as the relative contribution of
legume N sources to environmental losses of N from intensive pastoral systems.
The indirectly negative effect of N fertiliser use on legumes in a mixed pasture is well documented
globally (Matches 1979; Frame 1994; Eckard 1994; Whitehead 1995), as well as for Australian
conditions (McGowan 1987). However, little research has focused on the effects of strategic N
fertiliser use on clover content, especially where N fertiliser is applied only during periods of legume
dormancy, as recommended by Eckard (1996a) and McKenzie (1997). A recent analysis of dairy
farms in western Victoria has shown that the application of N fertiliser in spring (September) had no
effect on N2 fixation in October, or in the rest of the spring period (Riffkin et al. 1997). Simpson
(1987) reported a suppression of N2 fixation in legumes in the presence of high soil nitrate, but
indicated that this effect was seasonal and transient. This indicates that further research is warranted
on N fertiliser management aimed at the integration of clover and N fertiliser sources in intensive
pasture systems.
Recent surveys of grass clover ratios of farms in Victoria have shown alarmingly low clover contents
in pastures, being around 12% in the higher rainfall Gippsland regions and between 8 and 9% in the
drier west (Riffkin et al. 1997). Using 15N abundance analysis the average annual N2 fixation on three
farms in south western Victoria was estimated to be between 19 and 22 kg N/ha/yr (Riffkin et al.
1997). An earlier survey reported average clover contents of 15% in south western Victoria (Quigley
et al. 1992). Most studies agree that the clover content of a mixed pasture should be maintained at
around 30% to be of any significant plant and animal nutritional benefit (Walker et al. 1954; Roberts
et al. 1989; Whitehead 1995).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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One possible reason for loss of clover was alluded to by Ellington (1986), who discussed the
ecological niche of clovers as pioneer plants in an ecological succession. Particularly in conditions in
south eastern Australia, where most of the current pasture land was under some form of
sclerophyllous woodland 100 years ago, the virgin state of most soils would have been extremely low
in organic matter. With most of these areas having been sown to pasture between 80 to 100 years ago,
the soil organic matter status has improved substantially over time, to the point where clovers are no
longer in their ‘pioneer’ ecological niche, giving way to grass dominated pastures which are further
along the ecological succession continuum (Figure 7.). The accumulation of organic N under
pastures has been reported elsewhere (see Simpson 1987), as well as the role of clover in ecological
succession (Ball & Field 1987).
Figure 7. Diagrammatic representation of the succession development of a grassland system post
land clearing from scrub or forest (after Sears 1962, cited by Ball & Field 1987). Pasture
production is show as height above and soil fertility as depth below the base line.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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RESPONSES UNDER GRAZING
Most of the N response trials conducted in Victoria have been conducted in the absence of the
grazing animal, with comparatively few trials considering the effect of grazing on both nutrient
cycling and physical impact. In general, the few grazing trials reviewed by McGowan (1987) showed
responses of similar magnitude to the cutting experiments, a view supported by the data of Eckard
(1994). Rogers (1985) reported pasture N responses, recorded under grazing by dairy cows, of 15 to
23 kg DM/kg N applied. In these trials the application of 45 kg N/ha increased pasture growth rates
from 2 to 14 kg DM/ha/day in autumn and winter, and 17 to 76 kg DM/ha/day in spring. One trial
(Martin 1970, cited by McGowan 1987) showed small milk fat responses of 50g/kg N applied.
In assessing the economics of N responses McGowan (1987) assumed that the efficiency of N
fertiliser can be evaluated by ensuring that the ratio of kg animal product per kg N applied is greater
than the ratio of the price of N to the price of the product. However, to ensure that this is a fair
assessment pasture utilisation must be kept constant regardless of the rate of N fertiliser applied. In
other words, as N fertiliser increases pasture growth rate, stocking rate and N rate must be increased
to ensure that pasture utilisation is constant. The other issue is that there may be a negative effect of
high N fertiliser on animal performance and product, but a large increase in total animal product per
hectare. The above points are demonstrated in the data of Rogers (1985), where milkfat per cow
decreased with increasing stocking rate (Figure 8). However, the application of N fertiliser at the high
stocking rate substantially increased MF per hectare, while not changing MF per cow. A simple
calculation showed that this additional MF/ha made the use of N fertiliser highly profitable with a
0.59 kg MF/kg N response at an economic break-even of 0.29 kg MF/kg N. In another trial Rogers
(1985) reported an 8.3 kg DM/kg N response to 50 kg N/ha, applied in August, resulting in an extra
11 kg MF/cow and 41 kg MF/ha, with an effective animal response of 0.8 kg MF/kg N.
Figure 8. Relationship between average daily milk yield per cow over the first 6 weeks of
lactation and the application of N fertiliser (Rogers 1985).
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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The most comprehensive research on milk production, stocking rate and N fertiliser was conducted
by Rogers (1985) at the Ellinbank Dairy Research Institute (Figure 9). Both MF per hectare and per
cow increased with increasing N fertiliser rate. As stocking rate increased so MF per cow declined
while MF per hectare increased to an optimum stocking rate. At each stocking rate the application of
N fertiliser increased both MF per cow and per hectare. Rogers (1985) reported that the beneficial
effect of N fertiliser was mainly during early lactation, coinciding with early spring N responses.
Summer N responses were variable, with moisture being a likely limiting factor.
Figure 9. The relationship between stocking rate, milk production and N fertiliser application
(Rogers 1985).
The data of Rogers (1985) showed the main benefit of N fertiliser was achieved through an
increased stocking rate, and thereby the utilisation of the additional feed produced. In reviewing 5
trials, Rogers (1985) showed that, where stocking rates were low, MF responses to N fertiliser
were highly variable, but were more consistent where stocking rate increased with N application
rate.
The data of Rogers (1985) continued to show optimum stocking rates between 5 and 5.5 cows/ha
were required to adequately utilise a pasture fertilised with 100 to 200 kg N/ha.
The data reviewed by McGowan (1987) found little relationship between increasing N fertiliser rate
and animal production, mainly as the data were inferred animal production rather than trials
specifically designed to investigate such relationships. There is a dearth of data directly relating milk
production to N fertiliser rate on pasture from fully replicated experimental designs, mainly due to
the massive cost and resource demand of such trials. This is an area where simulation modelling may
be the only alternative.
Nitrogen for hay and silage
The economics of using N fertiliser to increase hay or silage has been debated extensively, with
arguments largely pivoting on the depreciation of equipment used and the true value of the forage i.e.
a drought year.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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Rogers (1985) compared the yield and feeding value of silage showing that N fertiliser did not affect
the feeding value of silage. More recent research in western Victoria (Jacobs et al. 1998) has strongly
supported the use of N fertiliser in terms of improved forage quality (discussed earlier). The
recommendations of both authors were that using N for a silage crop will improve the yield, thereby
increasing total silage or decreasing the area required, and it will allow for an earlier harvest and thus
earlier regrowth, returning the area to grazing sooner. Another ‘spin-off’ associated with the use of N
fertiliser for silage is that the farmer pre-plans the silage harvest, rather than the usual approach of
harvesting surplus grass that is rank and beyond its prime quality.
ENVIRONMENTAL IMPACTS AND LOSSES OF NITROGEN
Few data sets exist in south eastern Australia reporting the direct effect of N fertiliser on losses of N
from pasture systems.
Losses on N from legume based pastures
In 1986, Ellington reported that most of the studies conducted in Australia have not quantified the
losses of N associated with legume based pastures, as it is only in recent years that techniques for
measuring such losses have been refined enough to make reasonable estimates. However, the past 11
years have seen a marked increase in research focused on sustainable agricultural systems as well as
quantifying the environmental impacts of current practices.
Research on N cycling and loss process in New Zealand over the past 30 years have shown the
potential for substantial losses of N from intensively grazed pastures, particularly associated with the
spatial redistribution and concentration of nutrients in excreta (Ball & Field 1987; Simpson 1987).
Nitrogen removal
Nitrogen may be removed from the soil/plant system through the grazing animal or mechanical
harvesting. Estimates of N removal (after Ellington 1986):
1. Hay/silage = Yield x N content.
2. 300 kg/ha of lamb = 7.5 kg N (2.5% of live weight gain).
3. 9 kg N/ha from wool off 20 sheep/ha (0.11 kg N/kg wool).
4. Fattening stock retain 5 to 10% of N ingested.
5. Dairy cows retain between 15 and 25% of N ingested.
6. Nitrogen in milk: Friesian 0.53%, Jersey 0.61% (10 to 30% of total N ingested).
Organic nitrogen
Where the original organic N status of a soil is low, N generally accumulates in the surface soil, as
pastures develop (cf. Symbiotic N fixation; Simpson 1987). Accumulations of 40 to 80 kg N/ha/yr
may be expected from subterranean clover pastures, with higher values expected in wetter or
irrigation temperate areas (Simpson 1987). The rate of N accumulation and the time taken to
approach equilibrium depends on many factors including, other limiting nutrients, botanical
composition, climate, grazing management and the return or removal of nutrients (see Simpson
1987). A perturbation of the system, in any one of these factors, may initiate a shift to a new
equilibrium level of soil N. Intensive grazing has been shown to both increase total soil N, where
initial soil N is low, and decrease soil N where initial levels are high (Russell & Harvey 1959). High
grazing pressure tends to decrease overall soil organic N by spatially redistributing dung (Simpson
1987). Simpson (1987) concluded that it was difficult to control the accumulation of soil organic N,
as rainfall and initial organic N content are dominant factors in both mineralisation and
immobilisation. The above discussion does not account for the effect of N fertiliser coupled with a
high stocking rate, where it is suspected that the system would remain in a net positive N balance.
Legume tops were reported to decompose more rapidly than their roots, being correlated with the rate
of N loss from the legume (Amato et al. 1984). After 2 years, residual organic 15N from Medicago
spp. leaf, stem and root added to the soil amounted to 40, 56 and 50% of the N added respectively
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(Amato et al. 1984). After 4 and 8 years residual 15N was 45 to 50% and 28 to 35% of that added,
respectively (Ladd et al. 1985).
Mineralisation
Data from studies conducted in south eastern Australia appear limited. Ellington (1986) reviewed
some data from studies suggesting that earthworm populations may increase the rate of
decomposition of organic matter and, thereby, increase the rate of mineralisation.
In pasture soils mineralisation often keeps pace with immobilisation, due to the larger biomass
relative to cultivated soils, resulting in a net accumulation of organic N unavailable to plants
(Simpson 1987). However, only 1 to 3% of organic N in the soil is mineralised per year (Saffigna
1979; Ladd & Russell 1983). Thus many grass/ clover pasture systems may be in a negative N
balance if not fertilised with N (Simpson 1985). Vallis (1979) stated that mineral N is released when
the C:N ratio of residues is <20 to 30 or soil N content is >1.5 to 2.0% on an organic matter basis.
Atmospheric deposition
While estimates of atmospheric deposition have been made in many countries, data applicable to the
pastoral regions of Victoria were not found. Based on estimates from other countries Ellington (1986)
and Steele (1982) estimated inputs to be less than 10 kg N/ha/yr. However, these estimates could be
increased markedly in pastoral regions close to industry emitting ammonia i.e. the Latrobe valley coal
fired power stations.
Ammonia Volatilisation
Research on the volatilisation of ammonia from pastures in Australia was briefly reviewed by
Ellington (1986) and globally by Freney et al. (1981). Losses of ammonia can occur from growing
and senescent leaves, plant residues, soil surface litter, urine, dung, surface-applied N fertiliser and
through burning of dried herbage (Simpson 1987). There is general agreement that ammonia losses
can be large, particularly where animals are involved, as 60 to 90% (usually >80%) of the N ingested
is excreted as urea and nitrosamines, which are rapidly hydrolysed to ammonia (Simpson 1987,
Whitehead 1995). Galbally et al.(1980) estimated that around 26% of urine N excreted was lost
through volatilisation and that such losses were a function of stocking rate. Harper et al. (1983)
reported a 25% loss of urea applied to tropical pastures, based on 15N balance studies. In northern
Victoria, Mundy (1993) reported apparent losses of urea-N from pastures of between <10 to 32% of
the N applied, based on 15N balances. These apparent losses could be due to volatilisation,
denitrification and leaching, although up to 20% of the N was attributed to volatilisation.
Denmead et al. (1974) estimated annual losses from grazed lucerne at 100 kg N/ha/yr, while
Ellington (1986) estimated total Victorian losses at 39 to 74 Kt N/yr, or 500 Kt N/yr for Australia
(Galbally et al. 1980). Data from the ACT show ammonia losses from sheep grazed lucerne pasture
estimated at 0.26 kg N/ha/day, while Vallis et al. (1982) reported losses of 28.4% of the N applied as
simulated urine patches in southern Queensland. Using published figures Simpson (1987) calculated
potential annual ammonia losses from sheep grazed pastures between 40 to 60 kg N/ha/yr.
Large losses of ammonia are suspected from senescent plant residues but the processes are not well
understood or quantified (Simpson 1987). Vallis (1979) reported a loss of 20 to 40% of 15N applied
as residues on a soil surface, with evidence pointing to the gaseous loss of N from decomposing
surface residue. However, as living vegetation has been shown to absorb significant amounts of
ammonia through the leaves (Denmead et al. 1974; 1976; Whitehead 1995), and most pastures in
south eastern Australia are actively growing for at least 6 months of the year, the above estimates
could be halved (Ellington 1986). Packrou et al. (1997) reported volatilisation losses of 7 to 12 % of
applied 15N, from both irrigated and dryland pasture in South Australia during summer. Recycling of
ammonia within the pasture canopy can be a means of transferring fixed N from legume to grasses
under some conditions (Denmead et al. 1976), as well as a potential means of minimising losses from
urea fertiliser.
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The most recent research on ammonia volatilisation, in south eastern Australia was reported by Chen
et al. (1996). The study was conducted at Book-Book near Wagga-Wagga in New South Wales,
where ammonia volatilisation was estimated from annual and perennial pasture, with and without
lime. Under the conditions of the trial annual ammonia losses ranged from –7.9, for the annual
pasture without lime, up to 20.7 kg N/ha/yr from the perennial pasture with lime. Losses of 20.7 kg
N/ha/yr could be considered significant in a semi-extensive pastoral system where biological N2fixation forms the major input of N. However, in an intensive N-fertilised dairy pasture, such losses
would be considered negligible.
Ellington (1986) concluded that further research was required on volatilisation loss from pastures,
particularly the entrapment of ammonia by vegetation. Perhaps an area requiring further research and
investigation is the loss of volatile amines or aliphatic amines from grazed pastures; a loss process
almost certainly occurring but not yet quantified (Simpson 1987). Important Australian references to
volatilisation include Freney et al. (1981), Ellington (1986), Denmead et al. (1974), Denmead et al.
(1976), Galbally et al. (1980), and Simpson (1985).
Nitrate Leaching
The general opinion for Australian dryland pasture, according to Ellington (1986), is that leaching
losses are negligible, but would obviously depend on the climate, rainfall, soil hydraulic
characteristics and stocking rate (Simpson 1987). However, Ellington (1986) qualified this statement
by emphasising that little data were available to substantiate this claim and that estimates are often
speculative. Helyar (1976) considered that nitrate leaching must occur given the acidification of
pasture soils in Australia. This resulted in a series of trials aimed to quantify the nitrate leaching loss
from annual and perennial dryland pastures with estimates of 82 and 68 kg N/ha/yr being lost from
annual and perennial sheep-grazed grass/legume pastures respectively (Ridley et al. 1990). Pakrou et
al. (1997) reported nitrate leaching from grazed grassland to be a significant source of nitrate in
ground water in south Australia, however mean leaching losses were only 1 to 5 kg N/ha/yr, with a
peak concentration of 2.2mg N/L. Interestingly, Pakrou et al. (1997) reported ammonium to be the
major form of N leached, and was highest in the driest year of the study (1993). This was explained
by Whitehead (1995) as largely the leaching of urine (or mineral ammonia if at high concentrations)
down macropores, enlarged by the drier soil conditions.
Little consideration has been given to the potential for increased leaching of nitrate for intensive Nfertilised dairy pasture systems in the higher rainfall regions of south eastern Australia, nor to the
potential for runoff and erosion loss (Ellington 1986). Simpson (1987) eluded to the possibility of
nitrate leaching from pastures in the winter rainfall regions of south eastern Australia through the
mineralisation of organic N through the dry summer, when plant growth is restricted, and leaching
out at the onset of the ‘autumn break’.
The largest point source of nitrate available for leaching loss is through the spatial concentration of N
in urine (cf. N efficiency). In the absence of the grazing animal nitrate leaching losses from pasture
are minimal, even under relatively high inputs of N fertiliser (Whitehead 1995). In New Zealand
Sprosen et al. (1997) reported no difference in nitrate leaching losses from a grass/clover pasture and
a pasture fertilised with between 146 to 200 kg N/ha/yr, over a three year period. Schofield and
Tyson (1992) concluded that nitrate leaching loss could be greater from a grass/clover pasture, than
from a pasture fertilised with up to 200 kg N/ha/yr. In South Australia, Pakrou et al. (1997) reported
urine patches to be the dominant source of leached N, with nitrate concentrations often exceeding 60
mg N/L more than a year post grazing.
Although much research has been conducted in New Zealand, clearly there is a need for further
investigation of nitrate leaching from intensive, N-fertilised dairy pastures in south eastern Australia.
Denitrification /nitrification
Denitrification losses from intensively grazed pasture systems are suspected to be significant in south
eastern Australia (Simpson 1987). Extensive work on denitrification losses was conducted in
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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Australia by both Denmead et al. (1979) and Galbally et al. (1980). These data showed losses of N in
the range of 6 to 60 kg N/ha/yr from grass/clover pastures, with higher losses reported from flooded
conditions. Daily losses of 0.5 g N/ha, from a dry soil in winter, to 217 g N/ha, from a wet soil in
spring, have been reported (Denmead et al. 1979). Packrou et al. (1997) reported denitrification
losses of 12 % of applied 15N from irrigated pasture in South Australia during winter. In New Zealand
Ledgard et al. (1996) reported denitrification losses of 6 – 15 kg N/ha/yr, the range largely a function
of increasing N fertiliser use. Nitric oxide emissions have been associated with nitrite decomposition
(Galbally & Roy 1978) and would thus be expected from areas of high transient nitrite accumulation
(dung, urine, and N fertiliser). Vallis et al. (1982) found nitrite concentrations of 10 kg N/ha during
the seven days post urine application to pasture in south Queensland. Ellington (1986) concluded that
denitrification losses were likely to be highest when heavy rains fall on dead pasture in summer, or
when flooding occurred during winter. However, the associated low temperatures with the winter-wet
season in south eastern Australia may minimise the denitrifying effects of wet soil conditions.
Chen et al. (1996) reported on a study at Book-Book near Wagga-Wagga in New South Wales, where
denitrification rates were estimated from annual and perennial pasture, with and without lime. Under
the conditions of the trial annual denitrification losses ranged from 2.7 kg N/ha/yr, for the perennial
pasture without lime, up to 3.9 kg N/ha/yr from the annual pasture with lime. Losses of this
magnitude would not be considered agronomically or environmentally significant.
Ellington (1986) concluded that, although excellent research has been conducted on denitrification
losses, the data rarely cover long periods in time or a wide range of climatic events. Clearly further
research is required on denitrification losses from intensive N-fertilised dairy pasture in south eastern
Australia.
Surface run-off
No data were available reporting the loss of N due to surface run-off in dryland pasture.
Other loss processes
Some loss of N has been suspected due to erosion either through nutrients in surface run-off of water,
or through soil particles removed in wind erosion during dry spells (Simpson 1987). Simpson (1987)
also reported the burning of pasture and dead residues to produce NO, NO x and N2O, although little
data are available to quantify such losses locally.
A critical review of research on the nitrogen nutrition of dairy pastures in Victoria - R. Eckard (ISBN 0 7311 4270 5 October 1998)
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CONCLUSIONS
Based on the review of literature available the following conclusions can be drawn:
1. There appears some discrepancy in the shape of the response to N fertiliser, with most studies
showing some diminishing returns between 60 and 100 kg N/ha per application. However, there
are a number of studies, which report linear responses to N fertiliser in excess of 100 kg
N/ha/application. At the same time the international literature is reasonably clear that losses to the
environment become exponentially increased when the rate of N exceeds rates around 60 kg N/ha
per application. At this stage it appears that recommendations should place a maximum N
fertiliser rate at 60 kg N/ha/application, with a warning that exceeding this rate will increase
losses to the environment i.e. 50 kg N/ha on 2 ha will deliver a more predictable response that 100
kg N/ha on 1 ha.
2. It is accepted that higher rates of N (75 kg N/ha/application) may be useful if applied specifically
for silage yield with a longer regrowth period (6 weeks). This practice is well established in the
UK.
3. Some of the data indicate highly variable responses to low rates of N (below 25 kg N/ha), due to
the unknown contribution of clover to the nutrition of the pasture. Recommendations should
include some caution to this effect i.e. that 30 kg N/ha on 1 ha may consistently deliver a more
reliable response than 15 kg N/ha on 2 ha.
4. Based on 1 and 3 above it appears that N fertiliser rates should be between 25 and 60 kg N/ha
depending mainly on the quantity of additional feed required for a specified set of climatic and
edaphic conditions.
5. Responses to N fertiliser on grass clover pasture are highly variable when compared to data from
pure grass swards in the northern hemisphere. Some explanations for these variable responses
include:
a) The inclusion of clover in the pasture confounds many of the responses to N fertiliser due to
the average N content of the diet being higher (clover plus grass) and the often-unpredictable
temporal and spatial supply of N from the clover.
b) Being largely a winter rainfall climate one would expect highly variable N responses through
the summer when moisture supply may be limiting responses.
c) Being largely a 12 month growing season in Victoria a large proportion of data from the
northern hemisphere is not applicable, as winter grass growth is not possible. These differences
should be taken into account in translating findings.
d) In order to explain the variability in N response the clover content of the pasture, mineral N
content of the soil, together with seasonal and climatic conditions should be clearly
documented and included as covariates in any analysis of the data.
6. Utilisation of the extra forage produced is the key to the economic use of N fertiliser. Stocking
rate should increase with increasing use of N fertiliser on farm, otherwise efficient utilisation must
suffer.
7. Grazing studies conducted indicate that the application of N fertiliser either does not positively
increase yield or milkfat per cow but, as long as stocking rate is increased to utilise the additional
forage produced significant yield and milkfat yields will be achieved per unit area.
8. It appears that the response of 10 kg DM/kg N, as reviewed by McGowan (1987) is an average
over a range of conditions, and can clearly be exceeded most months of the year if N fertiliser is:
a) strategically applied to an adequately fertilised pasture,
b) of a highly responsive species composition,
c) in the recommended range of rates,
d) during a period when climatic and edaphic factors favour the response to N fertiliser.
e) A general rule-of-thumb is that the higher the growth potential of the pasture the greater the
potential response to N fertiliser.
9. Pasture height at time of fertilisation should be 5cm (1500 kg DM/ha) both shorter and longer
reduces responses.
10. Urea remains the cheapest source of N fertiliser. DAP is an even more economic source of N
fertiliser if P is required at the same time as N. At the current price of urea (91c/kg N @
September 1997) and assuming a minimum response of 10 kg DM/kg N and a maximum of 20 kg
DM/kg N applied, this would result in a cost per kg of dry matter in the range of 5.6 to 11.2 c/kg
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DM. Compared to the current cost of alternative feeds (i.e. grain at 22 c/kg DM) this represents a
significantly cheaper source of additional feed on farm. The above calculation excludes the cost of
additional P, K and lime that may be associated with N fertiliser use. The calculation also ignores
any carry-over effect of the N application into a second regrowth cycle.
11. Using the data from the review, plus data from current research a reasonably simple, yet generally
reliable Decision Support System could be developed. This process would assist in further
identifying essential data required.
Issues to consider for any future research
1. With most of the N responses quoted being measured by cutting herbage, utilisation by the grazing
animal is not built into the economic response rate. If a producer is not adept at managing a N
fertilised system wastage can be as high as 30 to 50%. This would convert a 10 kg DM/kg N
response into a 5:1 to 7:1 response, which would make the economics marginal.
2. Most studies base the harvest interval on either a set number of days throughout the year, or on
district average practice. In order to evaluate the true response to N, regrowth periods must be
based on some physiological growth stage of the plant, although this would mean that treatments
may have to be evaluated on different days.
3. It is vital that N response studies detail species composition and a complete soil analysis.
Environmental variables like soil temperature and rainfall are also important to allow a contextual
interpretation of the responses.
4. Any N response studies must ensure that N is the only limiting element, unless interactions with
other nutrients are part of the research.
5. The use of an N efficiency term (kg DM/kg N applied) may be misleading, as this does not
incorporate a time context. Perhaps this should be reported in kg DM/d/kg N applied, or growth
rate increasing/kg N applied.
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Issues requiring further research
1. The effect of higher rates of N fertiliser on animal production and health; particularly milk
production, nitrate toxicity and sub-clinical ammonia stress.
2. Most of the research conducted did not include dairy production responses directly. Even where
milk yield and milkfat responses are presented these are either inferred or derived from nonreplicated experiments (i.e Rogers). Some clarity is required as to the validity of such data or
inferred production before any replicated grazing trials are attempted.
3. There is much anecdotal evidence that the application if N fertiliser in mid-spring and summer
results in a suppression of yield in the regrowth phase following the initial response. While
explanations for such a suppression can be found, these effects have yet to be clearly proven and
quantified.
4. A much more ambitious and imaginative approach would be the breeding of grazing stock with
particular behavioural traits, which result in better excretal distribution (Simpson 1987). Research
in New Zealand is currently investigating ways of increasing the urination frequency of cows.
5. The loss of N from live and decomposing plant material (i.e organic N leaching) still requires
quantifying (Simpson 1987).
6. Research is still required to quantify the clover contribution under a range of strategic N fertiliser
regimes and the factors affecting clover activity in an N fertilised system i.e. clover content and
contribution to yield, available soil N, soil moisture content, soil temperature, and some of the
issues raised under point 4 of the conclusions.
7. Soil organic N accumulation under long-term permanent pasture, its effect on pasture composition
(clover content), soil acidity and the potential for a fodder crop niche to liberate the N stored in
organic form.
8. Research is required quantifying the N losses from a N fertilised dairy pasture system in south
eastern Australia viz ammonia volatilisation, leaching of nitrate, denitrification and surface runoff. Ellington (1986) concluded that “better estimates of N losses are required for each pastoral
system in south eastern Australia, with such estimates only being made from sophisticated
research on specific aspects of the N cycle”. Simulation models are the only practical option for
coping with the variability that exists in Australian dairy production systems. Models of the N
cycle in grazed pastures have been developed. However, these models need to be validated for
local conditions.
9. Interactions between nutrients have received little attention, particularly interactions between N
and P. As most P application rates are aimed at maintaining clover in the pasture, there may be
potential to exchange some of the P for N and achieve the same level of production. Perhaps some
correlation based on clover content, would be useful in determining the substitution level.
10. Modelling of the pasture response to N fertiliser, using soil temperature, daylength, season and
other environmental variables.
11. The relationship between water availability and pasture response to N fertiliser. A question
commonly asked by farmers is “how much rain is required in the summer before I can spread N,
and if so, how much N should be spread”? This could be the subject of a simple pasture growth
modelling exercise. The reverse question is posed in the winter when there is excessive moisture
available.
12. Estimating the true cost of N fertiliser use. This would include the cost of additional nutrients and
lime, as well as wastage/utilisation efficiency.
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