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Above-Ground Biomass Quantities and Livestock Production
at Big Sacaton Riparian Areas in Southeastern Arizona1
Jerry R. Cox and Howard L. Morton 2
Abstract.--Two big sacaton (Sporobolus wrightii) grassland riparian sites were studied in southeastern Arizona. At
site I we measured green biomass, dead standing and standing
crops of big sacaton for 3 years. At site II we annually
burned or mowed big sacaton pastures in February and annually
grazed these pastures plus an untreated control pasture between 1 May and 15 July for three years. Green biomass
peaked in August at 1300 and 3000 kg/ha in dry and wet years,
respectively. Dead standing biomass accumulated in the fall
and disappeared following either fall, winter or summer precipitation. Standing crop (green plus dead standing) was
greatest in August and averaged 4400 kg/ha. Both burning and
mowing reduced green biomass production. Stocking rates on
burned and mowed pastures were only one-third as high as on
untreated. Mean daily gains in 1981 and 1982 averaged 0.41
and 0.67 kg/day on untreated and treated pastures, respectively, but total gains per pasture were 512 and 235 kg on
the untreated and treated, respectively.
INTRODUCTION
The animal-carrying capacity of semiarid
grazing lands is dependent on the amount of plant
biomass which is available to be converted into
animal biomass. Therefore, a program to evaluate
carrying capacity should include studies to: (1)
quantify the annual accumulation and decomposition
characteristics of live biomass, dead standing
biomass and standing crop and (2) grazing studies
to relate plant and animal production.
Big sacaton (Sporobolus wrightii) grasslands
are found where floodwaters accumulate on alluvial
flats and flood plains in the southwestern United
States and northern Mexico. Because dead standing
biomass accumulates in these riparian zones land
managers have either annually burned or mowed in
late winter (February) and grazed in spring-summer
(1 May - 15 July) when upland grasses were dormant.
These treatments and spring-summer grazing have
been practiced for at least 100 years (Humphrey
1958).
The objectives of this study were to (1)
investigate the seasonal dyngmics of live biomass,
dead standing biomass and standing crop in a big
sacaton grassland and (2) evaluate the effect of
either annual winter burning or mowing and springsummer grazing of big sacaton on plant growth,
stocking rates and animal gains.
1 Paper presented at the North American
Riparian Conference. [University of Arizona,
Tucson, April 15-18, 1985].
2Jerry R. Cox is Range Scientist, Howard L.
Morton is Supervisory Plant Physiologist, U.S. Dep.
Agr. - Agr. Res. Serv., 2000 E. Allen Rd., Tucson,
AZ 85719. The authors' appreciation is extended to
the Donaldson Family at the Empire Ranch for their
cooperation, ANAMAX Mining Company for permission
to use their land and Reynaldo Madrigal for outstanding technical assistance.
STUDY SITES
Two sites representative of big sacaton grasslands
in the southwestern United States and northern
Mexico were selected about 80 km south of Tucson in
southeastern Arizona. Site I (Ecological Studies)
was in Gardner Canyon and Site II (Grazing Studies)
was in the Empire Creek drainage. Both sites are
within flood plains at an elevation of 1370 m, and
soils are classified as Pima silty clay loam, sandy
305
loam subsoil, thermic, Typic Haplustolls
(Richardson et al. 1979).
these assumptions: (1) additional plant growth
would not occur after 15 May or before the summer
rainy season began in mid-July, (2) each grazing
animal would eat or trample 10 kg of forage per
day, and (3) the desired utilization level when
animals were removed on 15 July would not exceed
60% of the standing crop present on 1 May.
Annual precipitation in the area has varied
from 175 to 450 rnm in the past 50 years (Sellers
and Hill 1974). Sixty percent of the annual precipitation usually comes in summer (June September) and 40% comes in winter (October April). Day-time temperatures average 30° C in
summer and night-time temperatures are often below
0° C in winter. Fall and spring are cool, dry and
windy (Cox 1984).
The standing crops of big sacaton live biomass,
dead standing biomass and other perennial grasses
were resampled on 15 May, 1, 15 and 30 June, 15
July and 20 October in 1980, 1981 and 1982. Sampling between 15 May and 15 July was to document
forage disappearance. The October date reptesents
peak standing crop after the summer growing season
(Cox 1984).
METHODS
Ecological Studies
A 2 ha study area was fenced to exclude livestock. Nine 15 x 15 m plots were established; 3
plots were selected and sampled at 6 week intervals
between 6 March 1980 and 6 February 1981, 3 were
selected and sampled between 6 March 1981 and 6
February 1982, and the remaining 3 were sampled between 6 March 1982 and 6 February 1983. Plant biomass within a 0.3 x 2.9 m quadrat was separated
into live (green) and dead standing (yellow and
gray) biomass. Standing crop was determined by
adding live and dead standing biomass components.
Study design, harvesting and statistical procedures
are detailed in Cox 1984.
RESULTS
Ecological Studies
Live Biomass
The amounts of live biomass were different at
the August sampling dates and similar at the remaining sampling dates over the 3 years (fig. 1). Summer thunderstorms began in early July and most of
August was dry in 1980 and 1982. Whereas, thunderstorms began in mid July and occurred regularly
through August 1981. As a result, live biomass was
2 to 3 times greater in August 1981 (3250 kg/ha) as
compared to August 1980 (1600 kg/ha) and August
1982 (1100 kg/ha).
Grazing Studies
A 16 ha study area was fenced and divided into
4 pastures. Three pastures were 5 ha and one
pasture was 1 ha. The three large pastures were
randomly assigned one of the following treatments:
(1) annual winter (February) burning and
spring-summer (1 May - 15 July) grazing (2) annual
mowing and spring-summer grazing and (3)
spring-summer grazing. The small pasture was retained as an untreated and ungrazed control.
3000-
*
L1ve
k
2000
1000
Live and dead standing big sacaton, and other
perennial grasses were sampled in each pasture on
27 February. One large pasture was selected as the
burned pasture and another as the mowed pasture.
The same pastures were burned or mowed in February
1980, 1981 and 1982. All pastures were sampled on
1 May and individually weighed Brahman heifers
(1980) and steers (1981 and 1982) released into the
three large pastures.
_g
~---f--1
\------~-.__1
0 L--,----,---,---,----,---,---,------,--
'
rJ>
rJ>
0
E
Standing Dead
0
iii
0
4000
~
3000
--~
2000
__ ,
-~/ "
t
Following late winter and spring burning and
mowing big sacaton grasslands in Texas produce
abundant green forage in late April and May (Gavin
1982, Haferkamp 1982). We expected a similar
response in southeastern Arizona, and stocking
rates in 1980 were based on expected growth rates.
Our assumption that rapid growth would occur in
April and May was incorrect because this period
corresponds to a drought season in Arizona, while
the same period corresponds to a wet season in
Texas. Therefore, the burned and mowed pastures
were over-stocked, and heifers were removed and
weighed on 1 June 1980.
1000
Apr
May
Jul
Aug
Oct
Nov
Dec
Feb
17
29
10
21
2
13
24
6
Sampling Dates
Fig. 1. Three year means and standard errors
(kg/ha) for live biomass, dead standing
biomass at 8 annual sampling dates for a big
sacaton grassland in southeastern Arizona. An
asterisk (*) above the standard error notation
indicates a significant difference (P = 0.05)
among years at the same sampling date.
Stocking rates in 1981 and 1982 were based on
306
Dead Standing
Pasture I ----Burning and Grazing
Pasture 2 ··••·•··· Mowing and Grazing
Pasture 3 ------- Grazing only
4500
The amounts of dead standing were similar in
October and averaged about 1750 kg/ha over the 3
years (fig. 1). There was significant variation in
the amounts of dead standing at the remaining
sampling dates over the 3 years.
4500
Pasture 4 - No treatment, No grazing
Dead standing averaged 3200 kg/ha during the
dry spring and early summer of 1980. About 50%
disappeared between August and October, but a
similar amount accumulated from the live component
in November. Approximately 45% of the amount that
accumulated in fall 1980 disappeared following
three snowstorms in January 1981.
[
4000
4000
3500
3500
3000
3000
en
en
2500
2500
E
0
CD
2000
2000
1500
- 1500
1000
1000
500
500
c
~
c
0
~
Dead standing averaged 1750 kg/ha during the
spring and early summer of 1981, and about 40%
disappeared following 145 mm precipitation between
10 July and 21 August. Dead standing began to accumulate in fall, and the total amount on 24
December was about 350% greater than on 21 August.
0
Dead standing averaged 3500 kg/ha during the
dry winter and spring of 1981-82. Approximately
25% disappeared in early summer when thunderstorms
began, and an additional 25% disappeared in late
summer when the thunderstorm activity resumed.
Cool-season precipitation in November and December
was 110 mm and standing dead averaged 2350 kg/ha.
This amount is about 1000 kg/ha less than on the
same dates in 1980 and 1981 when precipitation was
20 and 40 mm, respectively. Approximately 36% of
the dead standing present in December 1982 had
disappeared following three snowstorms in January
and one snowstorm in early February 1983.
Feb
May
May
Jun
Jun
Jun
Jul
27
I
15
I
15
30
15
Oct
20
Sam piing Dates
Figure 2. Three year means and standard errors
(kg/ha) for big sacaton dead standing biomass
at 8 annual sampling dates in four pastures.
Burning and mowing treatments were applied in
February; grazing was in spring-summer (1 May
to 15 July). Peak dead standing after the
summer growing season was measured in October.
Standing Crop
Standing crop, the sum of the live and dead
standing components, averaged 2350 kg/ha in
February and 4450 kg/ha in August over 3 years.
The percent of the live component within the
standing crop was less than 50% at all sampling
dates except two, 21 August and 2 October 1981.
3500
3500
Pasture I ---- Burning and Grazing
3000
3000
Pasture 2 ··· ······ Mowing and Grazing
Pasture 3 ------· Grazing only
c
......
Pasture 4 - No treatment, No grazing
2500
2500
~
Peak standing crops varied from 3900 kg/ha in
August 1982 to 5150 kg/ha in August 1980. Peak
standing crop of big sacaton was 4600 kg/ha in
southcentral Texas (Haferkamp 1982) and 4350 kg/ha
in west Texas (Gavin 1982).
en
en
c
E
CD
0
2000
2000
1500
1500
1000
1000
500
500
0
~
Grazing Studies
Burning and mowing immediately removed dead
standing biomass while grazing activities resulted
in a 45% decline between 1 May and 15 July (fig.
2). Dead standing removal during winter apparently
alters plant growth because live biomass in the
grazed pasture was about 45 and 35% greater than in
the burned and mowed pastures, respectively, on 1
May (fig. 3). These reductions in the available
forage resource following either burning or mowing
were reflected in pasture stocking rates in 1981
and 1982 (table 1).
Feb
May
May
Jun
Jun
Jun
Jul
27
I
15
I
15
30
15
Oct
20
Sampling Dates
Figure 3. Three year means and standard errors
(kg/ha) for big sacaton live (green) biomass
at 8 annual sampling dates in four pastures.
Burning and mowing treatments were applied in
February; grazing was in spring-summer (1 May
to 15 July). Peak live biomass after the
summer growing season was measured in October.
307
Table 1. Stocking rates, daily gains and total
pasture gains of Brahman heifers (1980) and
steers (1981 and 1982) grazing big sacaton in
southeastern Arizona. Burning and mowing
treatments were applied on 27 February 1980,
1981 and 1982 in the same pastures. Grazing
began on 1 May and ended on J June in 1980,
and on 15 July in 1981 and 1982.
Pasture I - - - - Burning and Grazing
'150 l
Posture 4 - - No treatment, No grazing
300
Year
1981
Treatment
Stacking Rate
(Head/ Pasture)
Daily Gain
(kg/ Animal)
Total Gain
(kg/Pasture)
0
350
Pasture 2 ·········Mowing and Grazing
Pasture 3------ Grazing only
300
250
250
200
200
150
150
100
100
.r;
.......
Burning and Grazing
12
.23
83
Mowing and Grazing
12
.23
83
E
Grazing
12
.45
162
iii
0
~
0
0
0>
..)/(.
1982
Burning and Grazing
Mowing and Grazing
Grazing
1983
Burning and Grazing
Mowing and Grazing
Grazing
5
.75
283
5
.67
250
15
.45
502
4
. 74
222
5
.50
187
18
.39
522
50
Feb
27
May
I
May
15
Jun
I
Jun
15
Jun
30
Jul
15
Oct
20
Sampling Dates
Figure 4. Three year means and standard errors
(kg/ha) for other perennial grass biomass at 8
sampling dates in four pastures. Burning and
mowing treatments were applied in February;
grazing was in spring-summer (1 May to 15
July). Peak other perennial grass biomass
after the summer growing season was measured
in October.
Heifers and steers preferred other perennial
grasses to big sacaton in the three large pastures
(figs. 3 and 4), and completely removed the
standing crop of other perennial grasses before
grazing big sacaton. The standing crop of other
perennial grasses was usually removed by 1 June.
Cattle began to graze big sacaton but were
constantly searching for other perennial grasses
between 25 May and 5 June. Between 15 June and 15
July animals grazed entirely on big sacaton.
DISCUSSION AND ECOLOGICAL IMPLICATIONS
Dead standing big sacaton biomass will rapidly
disappear following precipitation in either fall,
w~nter or summer.
These data do not support the
commonly accepted belief that dead standing biomass
disappears slowly under natural conditions
(Griffiths 1901, Humphrey 1960). However, the
logic which contributed to this belief is easily
understood. Dead standing biomass is the predominant vegetation component within big sacaton
grasslands for about 49 weeks of each year, and
even though dead standing does disappear after precipitation, it does not accumulate on the soil
surface between plants. Therefore, open areas
between plants are litter-free for the majority of
each year and this would suggest that most dead
standing biomass falls into the plant interior and
is trapped withi,l the remaining dead standing.
Average daily gains were highest in the burned
pasture, intermediate in the mowed pasture and
lowest in the grazed pasture in 1981 and 1982
(table 1). However, stocking rates in the grazed
pasture were 3 times greater than in the burned and
mowed pastures in 1981 and 1982, and total gains
were approximately 2 times greater in the grazed
pasture in all 3 years.
As dead standing disappears following either
fall or winter moisture, litter accumulates within
the remaining dead standing. This litter source
may serve as an important N reserve which becomes
quickly available for plant growth in summer
through the processes of decay, nitrogen mineralization and nitrification. The entrapment of N within
the remaining dead standing probably reduces N
losses associated with flooding.
308
Prior to channelization in big sacaton grasslands these riparian areas acted as a continuous
barrier that slowed floodwaters, trapped sediments
and enhanced the storage of soil moisture in
shallow water tables (Griffiths 1901, Hubbell and
Gardner 1950). Under these conditions the solum
remained wet for more than 90 continuous days
during the growing season (Soil Taxonomy 1979),
soils were classified as Haplustolls (Richardson et
al. 1979) and live biomass in the spring-summer
grazing period on areas which had been burned or
mowed in late winter was probably greater than on
untreated areas. After channelization, or
conditions which currently exist, upslope as well
as on site runoff quickly exits alluvial areas
through the channel system (Cooke and Reeves 1976),
the solum was dry during most of the growing season;
soils were reclassified as Torrifluvents
(Richardson et al. 1979) and live biomass was
greater in the spring-summer grazing period on untreated areas as compared to areas which had been
either burned or mowed in late winter.
The increase in daily animal gains would
suggest that big sacaton grasslands should be
either burned or mowed in late winter and grazed in
spring-summer. The increase in carrying capacity
on the untreated pasture would suggest that the
manager would most likely elect to decrease the
carrying capacity and increase daily gains. Therefore, the manager would burn because mowing is more
costly (Wright 1969) and then graze. The
alternative option of neither burning or mowing but
increasing the carrying capacity would be considered unrealistic because it is not logical to assume
that increased gain per area will compensate for
reductions in daily gains (Launchbaugh and Owensby
1978).
In the past big sacaton grasslands in lowland
and flood plains acted as barriers which slowed and
spread flood waters. Under such conditions it
would seem reasonable to assume that the live
biomass produced after burning or mowing would have
been greater .than on untreated areas because of the
extended growing season. These conditions no
longer exist and the presented data, collected over
3 years, would suggest that: (1) burning and mowing
do not stimulate live biomass production and (2)
annual burning and mowing may decrease live biomass
production under some conditions. Therefore, the
manager who is concerned with the long-term management of natural resources, rather than short-term
livestock gains, would likely discontinue the use
of late winter burning or mowing and increase carrying capacity during the spring-summer grazing
periods.
LITERATURE CITED
Cooke, R. U., and R. W. Reeves. 1976. Arroyos and
environmental change in the American southwest. Clarendon Press, Oxford.
Cox, J. R. 1984. Shoot production and biomass
transfer of big sacaton (Sporobolus wrightii).
J. Range Manage. 37:377-380.
Gavin, T. M. 1982. The effects of prescribed fire
on the production, utilization and nutritional
value of sacaton in Brewster County, Texas.
M.S. Thesis, Sul Ross State Univ., Alpine.
Griffiths, D.
Arizona.
Bull. 4.
1901. Range improvements in
USDA, Bureau of Plant Industry.
Haferkamp, M. R. 1982. Defoliation impacts on the
quality and quantity of forage harvested from
big sacaton (Sporobolus wrightii Monro). J.
Range Manage. 35:26-31.
Hubbell, D. S., and J. L. Gardner. 1950. Effects
of diverting sediment-laden runoff to range
and croplands. USDA Tech. Bull. No. 1012.
Humphrey, R. R. 1958.
Rev. 24:193-253.
The desert grassland.
Bot.
Humphrey, R. R. 1960. Arizona range grasses.
Arizona Agr. Exp. Bull. 298.
Launchbaugh, J. L., and C. E. Owensby. 1978.
Kansas rangelands; Their management based on a
half century of research. Kansas Agr. Exp.
Sta. Bull. 622.
Richardson, M. L., G. D. Clemmons, J. C. Walker.
1979. Soil survey of Santa Cruz and parts of
Cochise and Pima Counties, Arizona. National
Cooperative Soil Survey, USDA, Soil Cons.
Serv., Forest Serv. and Arizona Agr. Exp. Sta.
~'ashington, DC.
Sellers, W. D., and R. H. Hill. 1974. Arizona
climate (1931-1972). Univ. of Arizona Press,
Tucson.
Soil Taxonomy. 1979. A basic system of soil
classification for making and interpreting
soil surveys. USDA, Soil Conservation Serv.,
Agr. Handb. No. 436.
Wright, H. A. 1969. Effect of spring burning on
tabosagrass. J. Range Manage. 22:425-427.
309