2002 Unified Beef Cattle and Range Research Report (continued)
Effects of dormant season grazing on herbage production
and plant growth
Mitch Faulkner1, Kevin
Sedivec2, Jackie Olson2, Tim
Faller3, Jack Dahl4, and Greg
Lardy2
In May through November, seasonal forage availability declined
considerably between the time of peak production and the beginning of
the winter grazing season. Considering these losses, stockpiling of
forage throughout the growing season for use in late fall or winter resulted in
lost herbage production potential. Furthermore, AUMs/ha for
winter-only grazing areas were severely reduced relative to season-long grazing
use. Incorporating a brief early-summer grazing period on winter
pastures could increase land use and reduce economic loses by increasing
stocking rates (AUMS/ha).
From an ecological and land-use efficiency perspective, a
dormant season grazing system that incorporates moderate early summer
use combined with winter stocking rates utilizing 50% of the standing
plant biomass is a preferable, and moreover, a beneficial
management alternative. This method yielded greater herbage production than
other treatments and resulted in greater needle-and thread and
thread-leaf sedge leaf heights than the
season-long or DS 30 grazing treatments. This method, however, reduced
western wheatgrass leaf heights late in the growing season. If
dormant-season defoliation has little effect on
these grasses, limiting litter accumulation on stockpiled pastures by ensuring
at least moderate utilization (50%) of standing plant biomass may
positively affect subsequent herbage production. Furthermore, season-long grazing
may have a more negative effect on needle-and-thread and thread-leaf
sedge growth than winter use at higher (50%) utilization levels. The
direct effects of dormant-season grazing on individual plant species
versus conventional season-long use, at present, are undistinguished
in relevant literature. This research indicates that the four species
examined were generally unaffected by dormant season grazing.
Preliminary data regarding dormant-season grazing of native rangeland
in the western Dakotas indicated that brief early summer use of
dormant-season pastures and winter stocking rates intended to achieve
50% utilization of standing aboveground biomass is the preferred
management option relative to grazing treatments of 30 or 50% winter utilization
with no summer use. This method was beneficial from both a land-use
and ecological standpoint. Subsequent data are necessary; however,
to evaluate the long-term ecological and economic sustainability of
this management.
Introduction
Many North and South Dakota livestock producers practice winter or dormant-season
grazing in an effort to lower feed costs. Dormant season grazing, while not
an exclusive winter-grazing period, is defined as grazing during that time period
between plant quiescence in late fall and green up in early spring. Although
adequate information exists regarding nutritional management of winter grazing
cattle, little is known about the ecological effects of these practices "on
range or pasture land in the upper Midwest and northern Great Plains. Furthermore,
research-emphasizing inferences for specific winter-grazing management is lacking.
Various aspects of dormant season grazing have been examined in a variety of
ecosystem types, and conventional wisdom dictates that defoliation during winter
months while plants are dormant has little to no effect on plant vigor (Riesterer
et al. 2000).
Winter grazing is an appealing management option to many
ranchers. Producing hay or purchasing winter feeds is labor and
capital intensive, while winter grazing offers the potential for flexibility in
making management decisions. Furthermore, this practice allows for more
efficient utilization of range resources. The objectives of this study were
to determine the impacts of winter grazing on herbage
production, growth rate of dominant grass species (short-term), and changes in
plant species composition using various levels and combinations of winter
and summer use (long-term subsequent research).
Study area
This study was located in Adams County, North Dakota and
Perkins County, South Dakota. The Adams County study site was
approximately 153 acres and located 5 miles southwest of Hettinger, North Dakota
(El. 817m) on sections 16, T129N, R96W and 25, R97W, T129N. The
Perkins County study site was approximately 143 acres and located 16 miles
south of Lodgepole, South Dakota (El. 803m) on sections 13, T19N,
R12E, and 18, T19N, R13E.
Climate
Growing-season precipitation was 11.3 inches in 2000, which was
4.7 inches below the annual average, with all months except May and July
below average. The 2001 growing season was characterized as a dry spring
and wet July, with average precipitation 1.6 inches below the 30-year
average. The fall and winter of 2000-01 received above average
precipitation; however, the fall and winter of
2001-02 received considerably less precipitation, particularly in November
and December.
Monthly average temperatures were generally above the 30-year average
in 2000, with the exception of June, November, and December.
Warmer-than-average temperatures characterized the winter of 2001-2002,
as November and December 2001 and January and February 2002
were substantially warmer than the 30-year average. Spring and summer
temperatures were near average in both years.
Vegetation
The study areas were found in the northern mixed-grass prairie
and described as the Missouri Slope Vegetation Zone (USDA-SCS
1984). The plant communities were described as a
wheatgrass-needlegrass vegetation type (Barker and
Whitman 1994). Dominant midgrass species were western wheatgrass
(Pascopyrum smithii) and needle-and-thread
(Stipa comata), and dominant short graminoid species were
thread-leaf sedge (Carex filifolia) and blue
grama (Bouteloua gracilis) (Barker and Whitman 1994, Shiflet 1994).
Plant names were referenced from McGregor et al. (1986) and
USDA-USFS (2002).
Methods and design
Treatments
A total of two study areas (blocks) were selected in North and
South Dakota based on similar range condition and composition of
native plant species. Each study area was blocked and divided into four
paddocks with one of four treatments 1) season-long summer grazing at
50% utilization (SL), 2) 25% summer use for 2 weeks in early and mid June
and 50% dormant season utilization [flash grazing (Hart 2001)] (FL), 3)
30% dormant season utilization (DS 30), and 4) 50% dormant season
utilization (DS 50) assigned randomly to a paddock. The SL treatment was
an 80-acre paddock and the dormant season use treatments each
23-acre paddocks in North Dakota. The DS 30 and SL treatment paddocks
were each 30 acres in size, the FL treatment 37 acres, and the DS
50 treatment 48 acres at the South Dakota site.
Stocking rates
Stocking rates for the summer use treatments were determined using
the United States Department of Agriculture (USDA) Soil
Conservation Service (SCS) Technical Guidelines (1984) for the Missouri Slope
Vegetation Zone. Summer use paddocks were surveyed for ecological
site composition using the USDA SCS soil surveys for Adams County,
North Dakota (Ulmer 1987) and Perkins County, South Dakota
(Wiesner 1980). The stocking rate for the SL was calculated for a 4-month
grazing period beginning June 1 and ending October 1. The North Dakota
site was stocked at 1.9 ac/AUM with ten 1150 lb cows and their calves.
The South Dakota site was stocked at 1.6 ac/AUM with seven 620 lb
spayed heifers.
Summer grazing use of the flash grazing treatments (FL) was
targeted for 25% utilization. The FL treatment carrying capacity was calculated
by stocking for 50% use of the total available AUMs in June while
considering that 50% of the total annual production occurred by mid
June, thus achieving a 25% utilization of total annual biomass. The North
and South Dakota sites were stocked with ten and sixteen 1150 lb cows
and their calves or 4.4 ac/AUM and 4.1 ac/AUM; respectively, for two weeks.
Stocking rates for the winter grazing treatments were calculated
after determining dry-standing plant biomass on Nov. 15, 2000.
Ten randomly placed 0.25m2 frames were clipped for each ecological site
(n=2) existing within a given replicate (n=20). The USDA SCS
(Wiesner 1980, Ulmer 1987) soil survey maps and technical guides were used
to estimate ecological site composition within each paddock to calculate
total standing biomass. Final stocking rates for each treatment were computed
by calculating 25% grazing-use efficiency with 30 or 50%
disappearance, depending on treatment (Laycock et al. 1972, Pearson 1975) and a
dry matter intake for an 1150 lb non-lactating cow using the
National Research Council (1996) for beef cattle.
The North Dakota DS 50 and FL grazing treatment paddocks were
each stocked with four 1,150 lb cows, or 3.1 ac/AUM; and the DS 30
treatment paddock was stocked with three 1,150 lb cows, or 4.1 ac/AUM.
The South Dakota DS 50 treatment was stocked with 11 cows or 2.5
ac/AUM, the FL treatment stocked with 8 cows or 2.4 ac/AUM, and the
DS 30 treatment stocked with 6 cows, or 2.5 ac/AUM. All South
Dakota paddocks were stocked with cows weighing an average of 1150
lb.
Winter grazing cattle were allowed ad libitum access to white salt and
trace minerals and were supplemented with 3 lb/day on an as-fed basis of
30% crude protein all-natural cake. During the winter grazing period of
2000-2001, cattle grazed as snow cover allowed for 53 days beginning
November 15 on both the North and South Dakota study sites. During
the dormant-season grazing period of 2001-2002, cattle grazed on the
North Dakota site for 53 days beginning November 15. The cattle on
the South Dakota site grazed for 35 days and animal numbers were
increased to meet set stocking rate guidelines, as turn out
was delayed until January 12 due to mechanical failures
affecting the watering system.
Table 1 shows ac/AUM comparisons of treatments and percent change in carrying
capacities compared to the SL treatment (control). From a perspective of utilized
AUMs, the dormant season only grazing treatments reduced carrying capacities
relative to season-long use; however, the FL treatment numerically increased
carrying capacities slightly relative to season-long use (3.2 to 5.3%).
Table 1. Stocking rate comparisons among grazing treatments
in North and South Dakota.
----------------------------------------------------------------
SL FL DS 30 DS 50
----------------------------------------------------------------
N.D.
Ac/AUM 1.98 1.93 3.95 2.97
% Difference from SL 0.0 +5.3 -115.8 -61.8
----------------------------------------------------------------
S.D.
Ac/AUM 1.48 1.44 2.47 2.22
% Difference from SL 0.0 +3.2 -61.9 -38.1
----------------------------------------------------------------
SL = season-long summer grazing, FL = 25% summer use for 2 weeks
in early and mid June and 50% dormant season utilization,
DS 30 = 30% dormant season utilization, DS 50 = 50% dormant
season utilization
Herbage production
Herbage production of graminoids and forbs for each treatment was determined
using a paired-plot clipping technique (Milner and Hughs 1968). Twenty cages
were distributed in each pasture during the treatment period. One plot within
and outside each cage was clipped using a 0.25m2 quadrat. Clipped
herbage was separated into grasses and forbs, dry matter weights were recorded,
and lb/ac plant biomass and standard error of the mean were calculated for each
ecological site.
In the summer through winter periods of 2000-2001, five cages
were systematically placed on each of the two shallow ecological sites and
two loamy ecological sites before grazing began on each treatment (n=20),
with the exception of the South Dakota 30% treatment where only five
cages were placed on a shallow ecological site since this site made up only
10% of the study area on the treatment. During the winter of 2001 on
the North Dakota sites, five plots were clipped for both the loamy
and shallow ecological sites on the 30% treatment, five shallow plots
were clipped on the 50% treatment, and no plots were clipped on the FL
treatment due to ice and snow cover. On the summer treatments of 2001,
the 20 sites within each pasture selected for the tiller study were used
to determine production. In 2001-2002, all plots from the winter
grazing treatments were clipped since ice and snow cover did not prevent
clipping as it had in 2000-2001.
Leaf Heights
A study to examine leaf heights throughout the growing season was initiated
in May of 2001, to determine the growth patterns of western wheatgrass, needle-and-thread,
thread-leaf sedge, and blue grama within each treatment. The species were selected
as they were described as the predominant forage base of the study region (Barker
and Whitman 1994, Shiflet 1994). Furthermore, these species were described as
commonly existing together in various successional stages of rangeland in western
North Dakota (Hansen and Hoffman 1988). Goetz (1963) monitored the growth and
development of native range plants in western North Dakota and used leaf height
as a main indicator of plant growth. Furthermore, researchers have correlated
leaf and plant height with plant vigor, forage yield, competition, range condition
and trend, and defoliation levels (Short and Woolfolk 1956, Buwai and Trlica
1977).
Twenty locations indicative of the dominant forage base were
selected randomly within each treatment in May 2001. On each location, a
0.25 m2 quadrat was selected containing
at least 10 western wheatgrass tillers, five needle-and-thread tillers, 10
thread-leaf sedge tillers, and 10 blue grama tillers. Cool-season tillers
were marked with uniquely colored rings upon the selection of each site in
mid-May and each tiller was measured monthly until senescence was
observed for each species. Western wheatgrass and
needle-and-thread tillers were measured mid-month
for leaf height (height of tallest leaf) from May to August. Thread-leaf sedge
was measured mid-month for leaf height from May to July. Blue grama was
the only warm-season grass investigated for growth; thus, leaf heights
were measured mid-month during its growth period as described by
Goetz (1963), from June to September.
Statistics
A general linear model (GLM) was used to test for
between-subject effects for treatment-by-date
interactions of leaf heights for each species and herbage production.
When interactions were detected (P#0.05), treatments by date comparisons
were made using a GLM model to determine differences between
treatments and date. When interactions were not detected, data from all periods
and replicates were combined and a GLM model was used to determine
differences among treatments (P#0.05). Mean separations were performed
at P#0.05 using Tukey's Honesty Significant Difference (HSD)
procedure (Steele and Torrie 1980, SPSS 1990).
Results and discussions
Herbage production
No differences in herbage production were found between locations (P=0.296,
F=1.097) in 2000. Following one year of treatment, peak primary production on
the winter-only treatments did not differ (P>0.05) from the SL control treatment
(Figure 1). Furthermore, herbage production was higher (P#0.01) on FL than SL,
DS 30, and DS 50 after one year of treatment. No differences or positive effects
of moderate dormant season grazing treatments, similar to data reported by Coughenour
(1991) who found increased nitrogen in live and dead grasses and fringed sagebrush
on winter grazed areas, were found. Likewise, Schacht et al. (1998) observed
that mowing dormant range of switchgrass, little bluestem, and big bluestem
resulted in a higher yield of annual growth than a non-mowed control. Engle
et al. (1998) also reported that grazing strategies emphasizing defoliation
during the dormant season that decrease probability of multiple defoliations
during the growing season are less detrimental than those that increase the
probability of multiple defoliations, such as the FL treatment in this study.
Relevant research by Auen and Owensby (1988), Coughenour (1991), Engle et al.
(1998), Schacht et al. (1998) and Reisterer et al. (2000) indicate dormant-season
harvesting of grasses has little or no negative effect on subsequent herbage
production.
Figure 1. Peak herbage production on the summer grazed season long (SL),
June flash + 50% dormant-season use (FL), 30% dormant-season use (DS 30),
and 50% dormant-season use (DS 50) in 2001. Treatments with the same letter
are not significantly different (P>0.05). (Click here for a 24KB
black and white graph.)
Leaf Heights
No differences (P<0.05) in western wheatgrass leaf heights were detected
between treatments for the months of May and June 2002. In July, western wheatgrass
leaf heights in the DS 30 treatment were shorter (P<0.05) than the SL treatment
and in August both the FL and DS 30 treatment leaf heights were shorter (P<0.05)
than the SL treatment (Figure 2). Negative effects from grazing treatment on
late growing season plant production were also observed by Trent et al. (1988).
Fall grazed winter wheat plants relied more heavily on photosynthesis later
in the growing season than did the non-grazed wheat plants as they were unable
to draw from carbohydrate reserves during grain filling. Similarly, Buwai and
Trlica (1977) found heavy quiescent defoliation of western wheatgrass reduced
TNC relative to a non-defoliated control. Furthermore, moderate and heavy dormant
defoliation of western wheatgrass reduced both herbage yield and plant height
when compared to the control.
Figure 2. Western wheatgrass leaf heights on the summer grazed season
long (SL), June flash + 50% dormant-season use (FL), 30% dormant-season
use (DS 30), and 50% dormant-season use (DS 50) in 2001. Treatments with
the same letter within each month are not significantly different (P>0.05).
(Click here for a 38KB black and white graph.)
Light winter use (DS 30) resulted in shorter leaf heights than heavy
winter use for needle-and-thread, thread-leaf sedge, and blue grama. Light use
also resulted in lower needle-and-thread and blue grama leaf heights than
SL. These data suggest increased utilization during the dormant period
results in increased herbage yield the following year. Treatments by date
interactions were not detected (P<0.05) for needle-and-thread, thread-leaf
sedge, and blue grama; thus, monthly leaf height data were combined.
Needle-and-thread leaf heights throughout the growing season did not differ
between the SL, FL, and DS 50 treatments (P< 0.01); however, the DS 30 treatment
had lower leaf heights (P<0.05) than the SL treatment (Figure 3). Thread-leaf
sedge leaf height was also greater in the FL and DS 50 treatments (P<0.01)
than the DS 30 and SL treatments. The DS 30 and SL treatments did not differ
in leaf height (P<0.05) throughout the growing season (Figure 3). Blue grama
leaf heights did not differ (P<0.05) between the SL, FL, and 50% treatments;
however, the SL and FL treatments were higher (P<0.01) than the DS 30 treatment
(Figure 3).
Figure 3. Needle-and-thread, thread-leaf sedge, and blue grama leaf
heights on the summer grazed season long (SL), June flash + 50% dormant-season
use (FL), 30% dormant-season use (DS 30), and 50% dormant-season use (DS
50) in 2001. Treatments with the same letter within each grass species (a,b,c,
for needle-and-thread, k,l for thread-leaf sedge, and x,y for blue grama)
are not significantly different (P>0.05). (Click here for a 23KB
black and white graph.)
These findings are consistent with the peak herbage production
observations and studies by Coughenour (1991) and Manley et al. (1995) who
reported positive effects on herbage production with increased levels
of herbage removal during the dormant season. If dormant-season
defoliation is not detrimental to needle-and-thread, blue grama, and
thread-leaf sedge, removal of standing-dead plant material and the
corresponding reduction in litter on the soil
surface may be important to subsequent herbage production and plant
growth. Removal of standing dead plant material has been noted to elevate
soil temperatures; thus, accelerating decomposition and mineralization
in the spring. Furthermore, nutrient turnover rates are accelerated
under grazed systems by returning mineral nitrogen to the soil in a
readily decomposable form, thereby bypassing slower plant litter
decomposition pathways (Coughenour 1991).
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1Range Specialist, Bureau of Land Management, Lander WY
2Extension State Rangeland Specialist, Graduate Student and Extension State
Beef Specialist, Animal and Range Sciences Department, NDSU, Fargo
3Director, Hettinger Research and
Extension Center, NDSU, Hettinger, ND
4Range Specialist, US Forest Service, Dickinson, ND.
INDEX
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