Effects of selected conservation tillage practices on dry spell mitigation, productivity of water and cost
benefit
Mkoga, Z.J.a, Kihupi, N.a, Tumbo, S.D.a, Semoka, J.b
a
Sokoine University of Agriculture Department of Agricultural Engineering and Land planning, P.O. Box
3003, Morogoro, Tanzania
b
Sokoine University of Agriculture Department of Soil Science, P.O. Box 3003, Mororgoro, Tanzania
Corresponding author email: mkogazj@yahoo.co.uk
ABSTRACT
A considerable amount of knowledge on the advantages of conservation tillage practices as dry spell
mitigation and productivity enhancement measures exists.. However, there is still not much done especially
in semiarid Africa to elucidate soil water balance components and its effect on dry spell mitigation as
induced by conservation tillage practices (Hensley and Bennie, 2003). A field experiment was carried out in
the 2006/07 and 2007/08 seasons in Mkoji sub catchment of the great Ruaha river basin in Tanzania.
Conventional tillage (ox-ploughing crop residues removed) was comparatively evaluated with conservation
tillage practices of ox ploughing, tie ridging and ripping with surface crop residues and lablab dolicos cover
crop. Agricultural drought analysis in the 2006/07 season showed that a 14 days dry spell (69th and 83rd DAP)
in the ox ploughing bare (CT) soil treatment was reduced to 5 days (74th to 79th DAP) from effect of ripping
with surface crop residues (RR) treatments. Similarly, in the 2007/08 season dry spell of 25 days with CT
treatment was reduced to zero days with RR treatment. Analysis of productivity of water showed that
Ripping with crop residues with or without cover crop gave highest productivity of water of 0.5 and 0.6
kg/m3 respectively, compared to 0.3 kg/m3 with ox ploughing bare soil being an increase of about 50%.
Conservation tillage practice proved to be the most effective practice in runoff reduction if a ripping depth of
at least 25 cm is achieved. Similarly, a simple practice of leaving crop residues on the surface has a
significant effect in reducing runoff even with conventional tillage with ox-plough though not as effective as
for ripping and tie ridging. Ripping with surface crop residues with or without cover crop has are much more
effective in locations that receive seasonal rainfall between 300 and 700 mm.
Key words: Productivity of water, dry spell mitigation, conservation tillage, Soil moisture, rain water
reserve
1 Introduction
Rainfed agriculture in sub-Saharan Africa and Tanzania in particular, is a risky enterprise because it is
dependent on low and erratic rainfall and subject to long dry spells (Barron et al., 2003, Tilya and Mhita,
2007). Tanzanian economy is highly dependant on rainfall in which about 50% of GDP comes form rainfed
agricultural production (World Bank, 2005), and over 80% of the population live on agricultural earnings.
Based on agricultural production potential and rainfall analysis, it is estimated that more than one third of the
land in Tanzania is dry or drought prone. Semi aridity is characterized by low amounts of rainfall, high
evaporation rates, and poor temporal and spatial distribution of rainfall (Nieuwolt, 1973). The challenges are
to maximize infiltration, mitigate dry spells, and improve soil fertility management (Rockstrom et al., 2003).
Mkoji sub-catchment in the Great Ruaha River Catchment in Tanzania is no exception to this. The area is
comprised of semi-arid conditions subject to frequent dry spells causing great reductions in yields and
productivity of water. According to the 2002 national irrigation masterplan only 20% of the cultivated land in
Tanzania in under irrigation (JICA/MAFC, 2002) and therefore can have access to supplementary irrigation.
1
For example in Mkoji sub-catchment out of the cultivated 84,351ha only about 36% (30,270 ha) is under
irrigation (SWMRG-FAO, 2003). Ex-situ rainwater harvesting, which is another option for dry spell
mitigation is mainly practised in paddy growing areas by using bunds (majaruba) (SWMRG-FAO, 2003);
and very little if any in maize. On the other hand the benefits of conservation tillage agriculture as in-situ
rainwater harvesting in stabilizing and increasing crop production have not been well studied in Mkoji subcatchment (ARI Uyole, 2005). Even the research in locations outside Mkoji sub-catchment has not always
yielded consistent results in quantifying the amount of moisture conserved by the different conservation
tillage practices (RELMA, 1998). Most important is that not much has been done to explain the soil water
dynamics of the conservation tillage practices and how much soil water is potentially available to be
productively used by the crop, especially the potential of the practices in dry spell mitigation (Hensley and
Bennie, 2003; ARI Uyole, 2005). The overall objective of this study was to explore the potential of
conservation tillage practices in enhancement productivity of water through dry spell mitigation in Mkoji
sub-catchment. The specific objectives were (1) to assess the potential effects of selected conservation tillage
practices in runoff and infiltration, (2) Soil moisture conservation; (3) yield and productivity of water; (4)
cost benefit analysis.
2 Materials and methods
2.1 Experimental location, layout and design
A field experiment was conducted at the Ministry of Agriculture Training Institute Igurusi farm located
within ‘Igurusi ya Zamani’ Traditional Irrigation Scheme (IZTIS), representing the semi arid low lands in the
Mkoji sub-catchment and run for two rainy seasons of 2006/07 and 2007/08. IZTIS lies on latitude 8.330 S
and longitude 33.530 E, at an altitude ranging from 1100 to 1200 m above mean sea level. The location was
strategically selected to make use of the long-term climatic data at Igurusi agro-met station and to represent
typical soils under maize production in the semi-arid part of the sub-catchment.
Six tillage treatments were tested in the experiment as follows:
i)
Conventional tillage with ox-plough, with crop residues removed (CT),
ii)
Conventional tillage with ox-plough, with crop residues left on the soil surface (CTR),
iii)
Conventional tillage with ox-plough, with crop residues left on the soil surface and lablab
dolicos cover crop (CTRCv),
iv)
Tie ridging, with crop residues left on surface (TR),
v)
No till with ox-drawn ripper, with crop residues left on surface (RipR),
vi)
No till with ox-drawn ripper, with crop residues left on surface with lablab dolicos cover crop
(RipRCv).
The experiment was randomized complete block design (RCBD) with six tillage treatments replicated three
times. This resulted in 18 experimental plots of 4.5 m wide and 15 m long with length (67.5 m2) running
down the 2% slope.
2.2 Agronomic operations
Tanzania Maize Variety 1 – Streak Resistant (TMV1–ST) (Zea mays L.) composite maize cultivar was used
as a test crop being one of the maize varieties commonly grown in the study area. The crop was planted in
rows at spacing of 0.75 m between rows and 0.30 m between plants within a row in all the plots, which
resulted in 300 plants per plot (extrapolated plant population of 44,444 plants per hectare). Planting was done
on 21st December, and 15th January for the 2006/07 and 2007/08 seasons, respectively. Tillage was done 5
days before planting in each season. Di-ammonium phosphate fertilizer (N:P:K 18:46:0) was applied at the
rate of 60 kg P/ha and 54 kg N/ha at planting. Weeding was done twice over the growing season. First
weeding was about two weeks after planting while the second one was done when the crop stand was about
knee high. Top-dressing was carried out at five weeks after planting using Urea fertilizer at the rate of 66 kg
N/ha to bring the total nitrogen applied from the two fertilizer applications to 120 kg N/ha, as recommended
by Uyole Agricultural Research Institute for the study area (Mowo et al., 1993). At about five weeks after
planting an insecticide Actellic 50 EC was sprayed to control stalk borer infestation.
2
2.3 Soil moisture characteristics of the experimental site
A soil pit of about 1.0 m was dug at a representative location in the experimental site for soil description
according to FAO soil legend (FAO, 1999). Three soil horizons were observed from which core samples
were collected from each soil horizon and used to determine bulk density, and soil moisture characteristics.
Double plate pressure extractor was used to establish soil moisture at saturation, field capacity (0.001 bars)
and wilting point (15 bars).
2.4 Assessing runoff and soil moisture content
A standard rain gauge was used to record daily rainfall over the three seasons of the experiment. Burnt bricks
masonry wall of about 15 cm high was used to enclose each of the experimental plots. A slopping rectangular
trough, measuring 4.5 m long and 0.25 m wide was provided at the end of each experimental plot to collect
runoff water. Two collector concrete tanks placed in series and provided with multi-slot divisor system of
concrete tanks were used to collect and measure run-off such that one third of runoff water was collected in
the second tank. Soil moisture content was monitored in the experiment using an Institute of Hydrology
Neutron probe (Didcot 3965 NE Instrument Company, Abingdon). In the 2006/07 and 2007/08 seasons soil
moisture content measurements were taken twice a week, on Saturdays and Wednesdays. In case there was a
rainfall event reading was postponed to the next day. Two access tubes were installed at five meters apart
lengthwise along the centre line of each of the experimental plots. When taking the measurements, the probe
sensor was lowered through the access tubes to measure soil moisture content at 0-150, 150-300, 300-450,
450-600, 600-750, 750-1000 mm depths. The Neutron probe was calibrated for the experimental site at the
beginning of the experiment. The Neutron probe was calibrated for the 0-150 mm depth and for the 150-1000
mm depths as recommended by the manufacturers. Due to the presence of mudstones and gravels below the
one-metre profile depth in the fields, access tubes could not go below 1.00 m depth. Therefore soil moisture
was only monitored in the one-metre soil profile depth.
2.5 Crop development and yield
Crop development stages were recorded in four stages according to Doorenboos and Kassam (1979). The
stages were; establishment stage, vegetative stage, flowering stage, and grain filling stage. The duration of
growth stages were based on field observation of the crop development. The establishment stage was from
planting to 6-8 leaves formation. The vegetative stage was from crop establishment to tassel formation. The
flowering stage was from tassel formation to silking, and the grain filling stage included milk, dough and
dent stages of grain formation to physiological maturity. At harvest, seven middle rows in each plot
constituting an area of 2.5 m by 5.0 m (12.5 m2) were harvested by cutting the aboveground dry matter in
each plot and weighed. The seven middle rows were harvested in order to minimize border effect on the yield
results. After weighing the dry matter, the maize cobs were removed from the stalks, threshed and weighed to
obtain the grain weight. The grain moisture content at threshing was determined in the laboratory and the
grain yield was adjusted to 14% moisture.
2.6 Record of cost items
Cost items for cost benefit analysis of the different tillage practices were recorded. This included labour and
farm inputs. The duration involved in the different tillage treatments was measured using a stop watch. A
record of labour in mandays was determined as a combination of the number of people involved in the
operation and the duration of an activity. In case of labour input which could not be collected from this
research, data from past research work done in Tanzania was used. Most of the data was averaged from long
term research results of conservation tillage in Tanzania by Maganzu et al. (2007), Mkomwa et al. (2007)
and Ringo et al. (2007). Market prices were used to calculate cost of labour and other inputs. A simple
benefit cost analysis was done considering all costs and sales of grain yield at the prevailing market price.
3
2.7 Data analysis
Where applicable, data were subjected to analysis of variance (ANOVA) to test the effects of cumulative
runoff, above ground biomass yield, grain yield, and productivity of water based on the statistical model:
Yi jk    i  j  ij
Where:
Yijk = the responce
μ = the general mean
ρi = the effect due to block i,
αj = the treatment effect,
ωij = the main error
Mean separation was performed using Turkey test at 0.05 alpha levels. Results were portrayed in graphical
forms and tables.
3 Results and Discussions
3.1
Effect of tillage treatments on runoff yield
Results of the two year experiment showed consistent good effects of conservation tillage in moderating
runoff at the partitioning point in favour of mitigating agricultural dry spells. Table 2 shows variation of
cumulative runoff amount between tillage treatments in the two seasons of experiment. The figures in
brackets indicate the proportion of runoff as percentage of seasonal rainfall. Runoff yield differed
significantly (P<0.01) between treatments consistently over the two seasons (Table 2). Conventional practice
of ox-ploughing with removed crop residues gave the runoff of about 14% and 29% for the 2006/07 and
2007/8 seasons respectively, being significantly different from other treatments at P<0.01) at varying
magnitudes.
Table 2: Comparison of cumulative runoff (mm) for the respective tillage treatment during the three
experimental seasons
Tillage treatment
2006/07
2007/08
Ox-Ploughing
110.5a (14%)
181.2a (29%)
Ox-Ploughing crop residues
63.7ab (8%)
89b (14%)
Ox-Ploughing crop residues lablab
21.2bc (3%)
49.5c (8%)
Tie ridging
7.7bc (1%)
12.1f (2%)
Ripping crop residues
10.3bc (1%)
19.6d (3%)
Ripping crop residues lablab
4.4c (1%)
12.7e (2%)
Means in a column followed by different letters differ significantly at P<0.01 according to Turkeys’ Multiple
range test.
Cumulative runoff data showed that crop residues on the surface has a significant effect in reducing runoff
even if conventional tillage with ox-plough is practiced. This was indicated in the contrast between ox
ploughing bare soil and ox ploughing with surface crop residues for which there was a significant difference
between placement and removal of crop residues at 99% (P<0.01) level of significance. Where as
conventional tillage treatment with bare soil produced runoff ranging between 14% and 29%, placement of
residues reduced runoff to a range from 8% to 14% (Table 2). This is equivalent to runoff yield reduction of
between 42.8% and 51.7%. However this is practically difficult to implement because it needs to remove
crop residues before ploughing and put back after planting.
4
Three treatments, namely; tie ridging, ripping with crop residues and ripping with crop residues plus D.
lablab cover crop, showed the lowest and highly significant (P<0.01) runoff compared to ox-ploughing
treatments. Runoff produced in the three treatments maintained a low range of between 1% and 3%
throughout the three years of experiment. The effect of tie ridging in runoff reduction and moisture
conservation has been found in many other studies (Wilson and Gichuki, 1996; Kovar et al., 1992). However,
tie ridging involves high labour input in making them, and may be a reason for it not being very popular
among farmers with land in moderate slopes. Table 2 shows that ripping with surface crop residues can have
similar effect in reducing runoff as tie ridging. This was not expected because tie ridges are presumed to have
higher depression storage than ripping and thus higher effect in harvesting rain water. However, the two
passes of ripper applied in this experiment had a deep tilling effect (about 25 mm depth), which when
accompanied with crop residue mulch enhances ability to reduce runoff (Chiliba and Kabambe, 2003). The
conservation tillage practices are also important in mitigating effects of uneven distribution of rainfall in a
season. Figure 1 shows rainfall distribution in the respective maize crop growth stages for the 2006/07 and
2007/08 seasons. In the 2006/07 season the crop received no rain in the grain filling stage. This has effect in
reducing grain size and eventually grain yield (Kefale and Ranamakhaarachchi, 2004).
Cummulative rainfall (mm)
500
444.5
450
2006/07
2007/08
400
350
300
268
256
223
250
200
150
106
89
100
33
50
0
0
Establishment
Vegetative
Flowering
Grain filling
Crop growth stages
Figure 1: Distribution of rainfall received in the respective growth stages in the 2006/07 and 2007/08
experimental seasons
In the season 2006/07 also there was only 89 mm of rain received during flowering stage as opposed to crop
requirement for most maize cultivars. For example TMV1 requires about 136 mm of water for
evapotranspiration at flowering stage (Table 2). In such a season crop water requirement can only be satisfied
by the moisture reserved in the soil from the previous rainfall events. This is much more ensured if
conservation tillage practice is used. Figure 2 demonstrates the effect of conservation tillage practices on
reserving rain water to meet crop water demand in the different crop growth stages of TMV1 maize cultivar
in the 2006/07 and 2007/08 experimental seasons. It demonstrates the ability of conservation tillage
treatments to ‘reserve’ rainwater in the soil for the crop to utilise during the meteorological dry spells. For
example in the 2006/07 season there was a deficit during flowering (41 mm) and grain filling (88 mm)
stages, respectively of the incident rain to meet crop water requirement (Fig. 2). Similarly in the 2007/08
season rain water deficit of about 54 mm was recorded during the grain filling stage. In both seasons incident
rainfall was not sufficient to meet crop water requirement during the respective stage (Fig. 2). Conservation
5
tillage practices consistently maintained higher levels of water reserves by reducing runoff component of
rainwater partitioning.
For example in the 2007/08 season, while there was only 82 mm of seasonal rainwater reserved after meeting
crop water requirements in the CT treatment there was more than twice as much (213 mm, about 260%) as
water reserve in the RRCv treatment (Fig 2). This is a significant saving and can be very useful in extreme
weather conditions. The 2007/08 season received higher seasonal total (789 mm) which also recorded higher
rainwater reserves ranging between 279 (CT) and 379 (RRCv). A lower seasonal rainfall (630 mm) was
recorded with rainwater reserves ranging between 82 mm for CT and 213 mm for RRCv treatments in the
2006/07 season. Considering semi-aridity condition of the Mkoji sub catchment proportion of deep drainage
is negligible. Most of the reserve is therefore available to mitigate effect of dry spells. The bigger the
rainwater reserve the higher the ability of a tillage practice to mitigate effects of dry spells. The water reserve
is an insurance against intra-seasonal shocks of dry spells. This reserve water is also useful to maintain low
soil water matrix pressure necessary for water uptake by the crop and to support soil ecosystem processes
which keep the soil lively. This depends on soil moisture holding capacity which can also be improved by
adopting appropriate conservation tillage practice.
400
2006/07 season
Rain water reserve (mm)
300
373
330
368
346
376
279
200
100
0
-100
CT
CTR
CTRCv
TR
-200
6
RR
RRCv
250
209
Rain water reserve (mm)
2007/08 season
204
189
200
213
155
150
82
100
50
0
-50
CT
CTR
CTRCv
TR
RR
RRCv
-100
Tillage treatment
Establishment
Vegetative
Flowering
Grain filling
Seasonal reserve
Figure 2: Rainwater reserve in the respective crop growth stages based on effective rainfall after
subtracting crop water requirement (Table 4) and runoff (Table 5), indicating adequacy of the incident
rainwater in meeting crop water requirement for the TMV1 maize cultivar (CT = Ox ploughing bare
soil, CTR = Ox ploughing with surface crop residues, CTRCv = Ox ploughing with surface crop
residues and lablab dolicos cover crop, RR = Ripping with surface crop residues, RRCv = Ripping with
surface crop residues plus lablab dolicos cover crop).
3.2 Effect of tillage practices on mitigating dry spells
The two years’ rainfall and soil moisture data were used to show trends of both meteorological and
agricultural dry spells and the effect of the different tillage treatments in mitigating the intra-seasonal dry
spells (Figs 3 and 4)1. Generally, the trend showed that in all treatments soil moisture was well above wilting
point throughout the growing seasons indicating that there was no complete crop failure due to water stress.
However there existed some dry spells which might have some effects on crop performance. It was not
possible to explain the complete trend of soil moisture in all the crop growth stages because there were some
data gaps due to instrument failure in some days. For example in the 2006/07, no soil moisture data were
recorded between 1st and 14th days after planting, and 42nd to 65th day after planting. Similar data gaps were
recorded in the 2007/08. However it was possible to identify the occurrence of some meteorological and
agricultural dry spells using recorded data.
Rainfall trend shown in Figure 3 shows one meteorological dry spell exceeding 7 days in the 2006/07 season.
The dry spell spanned between 62nd and 70th day after planting (8 days). The remaining meteorological dry
spells were of less than 6 days which is considered to be less damaging. However further examination of
1
The soil moisture content data were plotted with soil moisture characteristic parameters (wilting point or the Lower
Limit - LL and field capacity or the Drainage Upper Limit - DUL) and the critical soil moisture content below which
crops start to experience soil moisture stress. The critical soil moisture is a cut point below which crop moisture stress
and so the agricultural dry spell begins. For the case of maize water stress begins at a depletion factor of 0.55 based on
the daily evapotranspiration of 5 mm/day (FAO, 1998). For the soils at the experimental site the field capacity (DUL),
critical soil moisture (0.55D) and wilting points were determined at soil moisture contents of 0.31 (mm/mm), 0.244
(mm/mm) and 0.19 (mm/mm), respectively.
7
rainfall pattern in this season showed that the period between 57th and 84th days after planting (24 days)
received light showers which could not meet potential evaporative demand of the atmosphere (Fig. 3). In this
period the effective rainfall of 45.7 mm was received while potential evapotranspiration demand was 114.1
mm, being more than twice as much as the rainfall amount. Similarly, there were two meteorological dry
spells in the 2007/08, one between 36th and 49th day after planting (13 days) and the other between 52nd and
61st day after planting (9 days), both during the vegetative growth stages (Fig. 4). However, between 34th and
50th day after planting (16 days) cummulative effective rainfall of 6.5 mm was received against the potential
evapotranspiration demand of 87.7 mm. If evapotranspiration demand was considered, the length of
meteorological dry spells would therefore be longer than normally conceived. Soil moisture in such periods
can drop to as low as to the critical soil moisture below which agricultural dry spell is eminent. Ability of
cover crop and crop residues to conserve moisture and mitigate effects of dry spells has been demonstrated
both in the 2006/07 and 2007/08 seasons (Figs. 3 and 4). In the 2006/07 season RRCv and RR treatments
recorded soil moisture at 0.269 (± 0.013) mm/mm and 0.257 (± 0.014) mm/mm, respectively and
significantly higher than in the CT treatments (0.247 ± 0.014 mm/mm) at P<0.01), about 9% difference.
Similarly in the 2007/08 season the RRCv and RR treatments recorded soil moisture of 0.263 (± 0.01)
mm/mm and 0.257 (± 0.014) mm/mm, respectively, which was significantly higher than that recorded in CT
(0.246 ± 0.008) at P<0.01, about a 6% difference. The effects of surface crop residues were evidenced from
the soil moisture trend in both Figure 3 and 4. The ox ploughing treatment with surface crop residues (CTR)
in both the 2006/07 and 2007/08 seasons recorded soil moisture content of 0.263 ± 0.002 mm/mm and 0.257
± 0.01 mm/mm which was significantly higher than 0.246 ± 0.003 mm/mm and 0.248 ± 0.01 mm/mm about
7% difference, respectively at P<0.01.
70
Daili rainfall: 2006/07 season
Rainfall/ ETo (mm)
60
50
40
30
20
10
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
0
Days after planting
Rainfall (mm)
Soil moisture comparison RR versus CT in the 2006/07 season
0.31
0.29
0.27
0.25
0.23
0.21
0.19
0.17
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
Soil moisture (mm/mm)
0.33
ET (mm)
Days after planting
CT
RR
RRCv
LL
8
DUL
0.55D
0.33
Soil mopisture comparison: CT versus CTR in the 2006/07 season
Soil moisture (mm/mm)
0.31
0.29
0.27
0.25
0.23
0.21
0.19
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
0.17
Days after planting
CT
CTR
CTRCv
LL
DUL
0.55D
Figure 3: Comparison of profile soil moisture between ripping and ox ploughing treatments in the
2006/07 season as affected by trend of rainfall storms. (CT = Ox ploughing bare soil, CTR = ox
ploughing with surface crop residues, CTRCv = ox ploughing with surface crop residues plus lablab
dolicos cover crop, RR = Ripping with surface crop residues, RRCv = Ripping with surface crop
residues plus lablab dolicos cover crop, DUL = Drainage upper limit, LL = Lower Limit, 0.5D =
Depletion at 0.5 of available soil water).
Rainfall/ ETo (mm)
60
Daily rainfall: 2007/08 season
50
40
30
20
10
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
55
57
59
61
63
65
67
69
71
73
75
77
79
81
83
0
Days after planting
Rain (mm)
Soil mois ture comparis on RR vers us CT in the 2007/08 s eas on
0.31
0.29
0.27
0.25
0.23
0.21
0.19
0.17
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
55
57
59
61
63
65
67
69
71
73
75
77
79
81
83
85
Soil moisture (mm/mm)
0.33
ET (mm/d)
Days after planting
CT
RR
RRCv
LL
9
DUL
0.5D
Soil moisture (mm/mm)
0.33
Soil mois ture comparis on: CT vers us CTR in the 2007/08 s eas on
0.31
0.29
0.27
0.25
0.23
0.21
0.19
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
55
57
59
61
63
65
67
69
71
73
75
77
79
81
83
85
0.17
Days after planting
CT
CTR
LL
DUL
0.55D
Figure 4: Comparison of trend of profile soil moisture over the 2007/08 growing season between oxploughing and ripping treatments with crop residues and cover crops as affected by rainfall pattern
(CT = Ox ploughing bare soil, CTR = ox ploughing with surface crop residues, CTRCv = ox ploughing
with surface crop residues plus lablab dolicos cover crop, RR = Ripping with surface crop residues,
RRCv = Ripping with surface crop residues plus lablab dolicos cover crop, DUL = Drainage upper
limit, LL = Lower Limit, 0.5D = Depletion at 0.5 of available soil water).
Although these differences look small they are sufficient to indicate treatment performances in term of soil
moisture conservation. The differences could be much more pronounced in locations with poorer rainfall
amount and distribution. More important was the effect of conservation tillage treatments in reducing periods
of agricultural dry spells. For example in the 2006/07 season a 14 days agricultural dry spell recorded
between 69th and 83rd days after planting in the CT treatment was reduced to about 5 days (74th to 79th DAP)
from effect of the conservation tillage treatments (RR, RRCv, CTR and CTRv) (Fig 5). Similarly in the
2007/08 season an agricultural dry spell of 25 days was recorded with CT treatment which was reduced to
zero with RRCv treatment and to 4 days with RR treatment (Fig. 6). Management practices, such as straw
retention, that alter evaporation or albedo may be the ones expected to change the soil water regime,
regardless of inherent soil characteristics ( Linden et al., 2000).
Soil moisture (mm/mm)
0.330
2006/07 season
0.310
0.290
0.270
0.250
0.230
0.210
0.190
0.170
64
69
74
79
84
Days after planting
CT
RR
RRCv
LL
DUL
0.55D
Figure 5: The effect of conservation tillage in mitigating effects of dry spell and reducing soil moisture
stress in the 2006/07 season
10
Soil moisture (mm/mm)
0.33
2007/08 season
0.31
0.29
0.27
0.25
0.23
0.21
0.19
0.17
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
Days after planting
CT
RR
RRCv
LL
DUL
0.5D
Figure 6: The effect of conservation tillage in mitigating dry spells and reducing soil moisture stress in
the 2007/08
3.3 Effect of tillage practices on maize grain yield and productivity of water
Table 3 shows a comparison of the effect of tillage practices on maize grain yield for the 2006/7 and 2007/8
seasons. Yield of maize ranged between 2.3 and 2.9 t/ha and between 1.7 and 3.8 t/ha, for the 2006/07 and
2007/08 seasons, respectively. Average grain yield was 2.7 and 2.8 recorded in 2006/07 and 2007/08 seasons,
respectively.
Table 3: Effect of tillage treatments on yield of maize for the three growing seasons
Yield of maize grain per season (t/ha)*
Treatments
2006/07
2007/08
Ox-Plough bare soil
2.84a
1.70b
Ox-Plough crop residue
2.86a
1.97ab
Ox-Plough crop residue lablab
2.74a
2.37ab
Tie ridge
2.69a
2.70ab
Ripping crop residue
2.30a
3.82a
Ripping crop residue lablab
2.39a
3.82a
Coefficient of variation
14.4%
19.1%
Average
2.71
2.75
Seasonal rainfall (mm)
789.5
630
*Means in a column followed by different letters differ significantly at P<0.05 according to Turkeys’
Multiple range test
There were no significant differences in yield between treatments (P<0.05) in the 2006/07 probably due to
sufficient rainfall in the season which was better distributed than in the 2007/08 season. Although there was
no rainfall received during grain filling stage in the 2007/08 (Fig 1), there was equally a high rainwater
reserve (Fig 2) to mask the rain water deficits recorded during flowering and grain filling stages. This was
adequately explained by the soil moisture trend in Figure 4 which recorded only a slight soil moisture stress
in the CT treatment during flowering stage.
11
Grain yields in the 2007/08 season differed significantly from each other at P<0.05. This may have been
attributed to insufficient rainfall during the crop establishment stage. Figure 7 shows that the 2007/08 season
received less rainfall to support crop establishment. It was found that up to 8 Days after planting (DAP) only
4 mm of rain were received in 2007/08 season compared to 113.5 mm for the 2006/07 season. Even after 14
DAP 2007/08 crop received only 44 mm as compared to 210 mm for the 2006/07 season. Further analysis
showed that about 60% of cumulative rain (i.e.106 mm) which was recorded (between 0 and 23 DAP) in
2007/08 season, fell in the last seven days of crop establishment stage well after seed germination and
seedling emergence stages have elapsed. This must have resulted in reduced water supply and poor crop
establishment in tillage treatments which had low ability to conserve moisture. This could be explained by
the amount of soil moisture at the seeding zone (about 50 mm depth). However such data could not be
collected because it was beyond the scope of this study. Available data relate to an average soil depth of 150
mm which cannot explain precisely the soil moisture condition at the seeding zone.
300
Cummulative rain (mm)
250
2006/07
2007/08
200
150
100
50
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
Days after planting
Figure 7: Comparative trend of cumulative rainfall during crop establishment stage between the three
seasons
Ripping with crop residues and ripping with crop residues and D. lablab cover crop, yielded 3.8 t/ha which
was significantly higher than the other tillage treatments (P<0.05). The effect of insufficient rainfall during
crop establishment stage discussed above may explain this phenomenon. Germination difficulties and low
plant densities were observed at the beginning of the 2007/08 season and recorded as plant stand 21 days
after emergency. Figure 8 shows variation of number of plants established per unit area in the 2006/07 and
2007/08 seasons. There was no significant difference in plant establishment in the 2006/07 season. In the
2007/08 season more plants per unit area were established in tillage treatments which have ability to conserve
soil moisture. In this season both ripping with residues and with or without cover crop treatments recorded
2.1 and 2.9 plants/m2, respectively, being significantly higher than the rest of the treatments (P<0.05). This
may have caused the ripping treatment coupled with crop residues application to record highest and
significantly different yield (P<0.05) from conventional tillage with ox-plough. This is further related to
treatments with high ability to mitigate dry spells. The number of established plants closely correlated with
final recorded yield for each plot. Severe water deficit is common during crop establishment in many areas of
the semi-arid tropics. Results of this research suggest that use of crop residue mulch can help in conserving
12
soil moisture needed for enhancing germination, maintain good plant population and high yield. Abrecht and
Bristow (1996) argues that maintenance of soil cover in close proximity to the emerging seedling slows soil
drying, thereby improving water availability to the seedling, delaying the onset of high soil impedance and
reduces maximum soil temperature. The overall effect is to increase both the rate of emergence and
proportion of seedlings that emerge which relates to final grain yield.
3.5
Plant density (Plants/m2)
2006/07
2007/08
2.9a
3
2.9a
2.5
2
1.5
1.7b
1.3b
1.8b
1.4b
1
0.5
0
CT
CTR
CTRCv
TR
Tillage treatments
RR
RRCv
Figure 8: Variation of plant population 21 days after sowing between the different tillage treatments in
the 2006/07and 2007/08 seasons (CT = Ox ploughing bare soil, CTR = ox ploughing with surface crop
residues, CTRCv = ox ploughing with surface crop residues plus lablab dolicos cover crop, TR = Tie
ridging, RR = Ripping with surface crop residues, RRCv = Ripping with surface crop residues plus
lablab dolicos cover crop)
Table 4 provides an overview of productivity of water in terms of mass produced per unit of rain water
received during the crop growth period. There is a similar trend of productivity of water in the 2006/07 and
2007/08 seasons as that of grain yield discussed above. The highest (0.6 kg/m3) and least (0.3 kg/m3)
productivity of water were obtained in the seasons. There was no significant difference (P<0.05) in
productivity of water between treatments in the 2006/07 season. Productivity of water in the second season
differed significantly (P<0.01) between treatments. Ripping with surface crop residues and Dolicos lablab
cover crop recorded the highest productivity (0.6 kg/m3) and significantly different from all the other
treatments. Conventional tillage produced maize grain of 0.3 kg/m3 the ripping with surface crop residues
and with D. lablab produced 0.5 and 0.6 kg/m3, respectively in the 2007/08 season. This was equivalent to an
increase of about 67% to 100% with RR and RRCv treatments, respectively, compared to CT treatment. The
productivity values reported here relate to those reported by other workers in Mkoji sub catchment and
elsewhere. Recent research recorded productivity of water for maize in rainfed agriculture was 0.19, 0.27 and
0.66 kg/m3 in the middle, upper and lower Mkoji sub-catchment, respectively (SWMRG-FAO, 2003).
Igbadun et al. (2005) study with official production data from Mbarali district Mbeya Tanzania, reported
maize grain productivity ranging between 0.19 and 0.49 kg/m3 based of total seasonal rainfall.
Rao and Okwach (2005) reported productivity of water for maize grain of 0.3 kg/m3 and 0.12 kg/m3 in the
normal and above normal years, respectively from a study in Machakos, Kenya. The reported water
productivity for the sub Saharan Africa ranges between 0.1 and 0.6 kg/m3 (Rosegrant et al., 2002). The
productivity values reported in this study are therefore within reasonable range as found by other studies.
However, high levels of productivity are closely associated with high fertility use because, water saving
13
investment alone is unlikely to give potential productivity benefits because low soil fertility is a fundamental
constraint in smallholder agro ecosystems unless coupled with nutrient management (Twomlow et al., 2006,
Rockstrom et al., 2007).
Table 4: Effect of tillage treatments on physical productivity of water for the three growing seasons
Productivity of water based on seasonal rain (kg/m3)
Treatments
2006/07
2007/08
Ox-Plough bare soil
0.3a
0.3b
Ox-Plough crop residue
0.3a
0.4ab
Ox-Plough crop residue lablab
0.4a
0.4ab
Tie ridge
0.3a
0.4ab
Ripping crop residue
0.3a
0.5a
Ripping crop residue lablab
0.4a
0.6a
Coefficient of variation
14.4%
19.1%
Probability
P<0.05
P<0.01
Average
0.34
0.43
Total rain during growth season (mm)
789.5
630
*Means in a column followed by different letters differ significantly at the indicated probability according to
Turkeys’ Multiple range test.
3.4 Comparison of benefits and costs between tillage practices
A comparison of simple benefit cost analysis between conventional ox-ploughing, tie ridging and ripping
with crop residues over the two seasons consistently showed advantages of conservation tillage. Conservation
tillage practice (i.e. ripping with crop residues) was most profitable in the two seasons. It gave highest returns
for every unit input of crop production as shown by the benefit-cost ratio (Fig. 9). Tie ridging also showed a
consistent ability to give profit by conserving moisture even in a poor season. However the returns were
smaller per unit of input (Fig. 9). The major problem with tie ridging is the high labour input required
annually for making the ridges. It was found in this research that a total of labour input of about 134
mandays/ha/year is required for maize production based on tie ridging (Table 5). Ripping treatment had a
labour requirement of only 31 mandays/ha/year which accounts for labour saving of about 70%.
The most sensitive factors affecting benefits in maize production include yield fluctuations and cost of inputs.
Conservation tillage practice maintained high yield throughout the two seasons, while conventional oxploughing did not. Also there was a sharp rise of price of fertilizers over the years. For example a 50 kg bag
of basal fertilizer, Di-ammonium Phosphate, was costing Tshs 30,000 at the beginning of 2005/06 season.
The price rose to Tshs. 100,000 in the 2006/07 and 2007/08 seasons. Therefore, conservation tillage practices
have high ability to cushion effects of both yield fluctuations and cost of inputs by mitigating bad effects of
intra-seasonal dry spells.
14
2
2006/07
1.5
2007/08
Benefit cost ratio
1
0.5
0
Conventional tillage
Tie ridging
Ripping with crop residues
-0.5
-1
Treatments
Figure 9: Comparison of benefit cost rations between conventional and conservation tillage practices at
Igurusi
Table 1: Comparison of labour requirement (mandayS/ha) between conventional and conservation
tillage practices
Treatment
Requirement
Labour saving/loss
Conventional tillage
Tie ridging
104
134
0.00%
-37.50%
Ripping with crop residues
31
70.20%
4 Discussion
Tillage practices such as ripping have considerable effect in runoff reduction and soil moisture conservation
in a similar level as tie ridging (Table 2 and Fig. 2). The effect was much more shown in Figure 2 in which a
concept of rain water reserve is demonstrated. The rainwater reserve concept has been used to present the
effect of ripping, crop residues and cover crops in harvesting runoff and maintain high soil moisture used to
bridge the intra seasonal dry spells, maintaining high yields and productivity of water. In a way it has been a
tool to exhibit the ability of conservation tillage practices in mitigating meteorological and agricultural dry
spells by harvesting incident rainwater keeping it in the soil for the subsequent use during the dry spells.
However crop residue mulch seems to have bearing in augmenting the effects of tillage on runoff and soil
moisture. This was shown by its effect in reducing runoff (Table 2) and maintaining higher soil moisture
(Fig2) even with conventional tillage practice. In a recent study Enfors (2009) found that ripping alone
without surface mulch provide very temporary effect on soil moisture and that mulching has an additional
15
effect in decreasing soil evaporation and increase infiltration. The advantage of ripping over tie ridging is
exhibited also in terms of labour saving (Kaumbutho and Kenzle, 2007; Shetto and Owenya, 2007), and
higher cost benefit ratio. There is also the advantageous effect of biological breaking of plough induced soil
hardpan by means of deep growing tap root of cover crops (ARI Uyole, 2005).
However, management practices such as conservation tillage do not result in consistent effects on
productivity for all soils and environments (Zobeck et al., 2007). For example, in locations of insufficient
rainfall such as Makanya, Same Northern Tanzania, which receives seasonal rainfall of less than 300 mm,
Enfors (2009) found no in-situ rainwater harvesting effect in seasons with poor distribution, but realised
some positive effects in a good rainfall season (549 mm) in terms of yield increase. Therefore the effect of
conservation tillage is well realised if incident total rainfall is sufficient to meet seasonal crop demand. For
example farmers in Mayale village, Iringa Tanzania were able to harvest maize from ripped plots in 2001
when rainfall was 560 mm (Mkomwa et al. 2007). The same was reported in Karatu, Arusha, Tanzania in
locations with rainfall range of between 300 - 700mm (Ringo et al. 2007).
5 Conclusion
Conservation tillage practices which involves components of ripping, crop residues and cover crops have
significant effects on absorbing weather related shocks shown by consistent effects on reduced runoff,
maintaining high soil moisture regimes, improved yield, high productivity and high profits realised even in
bad seasons where conventional tillage practices have recorded severe economic losses. Use of conservation
tillage helps in conserving soil moisture needed for enhancing germination; maintaining good plant
population and yield, thus mitigating severe water deficits during crop establishment common in many areas
of the semi-arid tropics. Ripping with crop residues with or without cover crops has the highest effect in
runoff reduction and about the same effect in runoff reduction as tie ridging if ripping is done at least to 25
cm depth. Ripping with surface crop residues with or without cover crop has are much more effective in
locations that receive seasonal rainfall between 300 and 700 mm.
Acknowledgement
I acknowledge the Comprehensive Assessment Programme of the International Water Management Institute
(IWMI) for the financial support through the Soil Water Management Research Group (SWMRG) of Sokoine
University of Agriculture. I also acknowledge my employer, the Ministry of Agriculture, Food Security and
Cooperatives, Tanzania for complementary funding of this study.
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