Advancing Water Resources Research and Management
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| Proceedings: Specialty Conference on Rangeland Management and Water Resources |
Abstract
Introduction
Materials and Methods
--Study Site
--Experimental Design
--Above Ground Biomass Production
--Soil Water
Results and Discussion
--Above Ground Biomass Production
--Soil Water
Conclusions
References
Acknowledgements
Changes in available soil water and biomass production on overflow and silty range sites in response to no grazing, light, moderate, heavy and extreme grazing were monitored for nine years in southcentral North Dakota. Each treatment was replicated three times. The pastures were stocked to leave 65%, 50%, 35% and 20% of an average year's above ground biomass remaining at the end of the grazing season on the light, moderate, heavy and extreme treatments, respectively. Soil water was sampled approximately every two weeks through the growing season. Biomass production was sampled at the beginning of the grazing season, peak of biomass production, and end of the grazing season. On overflow range sites available water has tended to be less on the sites which were most heavily grazed. On silty range sites the moderately grazed treatments have tended to have more available water than ungrazed or extreme grazed treatments. Significant differences have occurred during both periods of soil water recharge and discharge indicating increased runoff and evaporation from the soil surface on the heavily grazed treatments. Plants on the ungrazed treatment on silty sites had more leaf area than plants on the moderate treatment and appear to remove more water through transpiration. Biomass production has been reduced on both the extreme and ungrazed treatments compared to the moderate treatment on both silty and overflow range sites.
The Great Plains grasslands evolved under grazing by large herbivores (Lauenroth et al. 1994). The individual plant species which make up the grassland plant communities vary in their adaptive mechanisms and tolerance for grazing so the composition of the community will shift over time in response to different grazing intensities (Biondini and Manske 1996, Brand and Goetz 1986, Hart et al.1993). However, there is some debate about the effect of grazing on biomass production (Maschinski and Whitham 1989, Williamson et al. 1989). Under heavy grazing will the plant communities produce less biomass, or will palatable species just be replaced with less palatable species? Will the most biomass be produced with no grazing, or will a buildup of dead material reduce growth or alter nutrient cycles? The amount of vegetation on the ground should affect water infiltration and runoff, and evaporation and transpiration. This in turn will effect the amount of water available for plant growth (Branson et al. 1981).
Livestock producers are generally concerned with optimizing production on their grazing land. An obvious question is how heavily can the land be grazed without seriously damaging its long term productivity. This study looked at the effect of varying grazing intensities on available soil water and biomass production.
Study Site
The study was conducted at the Central Grasslands Research Extension Center (CGREC) near the eastern edge of the Missouri
Coteau about nine miles northwest of Streeter, North Dakota. The study site is typical of rangeland in the Missouri Coteau which
consists of a mosaic of soil types and range sites. Silty range sites (nearly level to rolling uplands, with slopes from 1 to 15% and
deep, moderately well drained to moderately fine textured soils) and overflow range sites (nearly level to gently sloping lands which
receive run-off water from higher sloping lands, with deep, well aerated sandy loam to clay textured soils) dominate the study site.
Elevations range from 1900 to 1970 ft. The climate is continental, the average annual precipitation is 17.85 in., 72% of which is
received during the growing season (May through September). Table 1 shows the annual precipitation and above ground biomass
production averaged over the five treatments each year of the study.
Until the current study began the area had been lightly grazed by livestock and wildlife. The vegetation is mixed grass prairie. Silty sites are dominated by Kentucky bluegrass (Poa pratensis L.), green needlegrass (Stipa viridula Trin.), sun sedge (Carex heliophila Mach.) and western wheatgrass (Agropyron smithii Rydb.). Overflow sites are dominated by Kentucky bluegrass, smooth brome (Bromus inermis Leyss.), western snowberry (Symphoricarpos occidentalis Hook.) and rigid goldenrod (Solidago rigida L.). Nomenclature follows Flora of the Great Plains (Great Plains Flora Association 1986). The balance of the vegetation is composed of a very diverse assemblage of grass and forb species most of which are readily accepted by grazing livestock.
The experiment began in 1989 as a completely randomized design with five treatments: light, moderate, heavy and extreme grazing intensities and an ungrazed control. Each treatment was replicated three times on pastures of about 32 acres each. Sample sites on silty and overflow range sites were selected in each pasture. Exclosures (66 ft x 394 ft) were built on three silty and three overflow range sites to provide the ungrazed control. The pastures are stocked in mid to late May with the goal of leaving 65% (2,083 lbs/ac), 50% (1,652 lbs/ac), 35% (992 lbs/ac) and 20% (505 lbs/ac) of an average year's above ground biomass remaining at the end of the grazing season on the light, moderate, heavy and extreme grazing treatments, respectively.
The goal of leaving a certain percentage of average year's above ground biomass was chosen to try and maintain a constant grazing
pressure on the plant communities despite fluctuations in production. This required varying the length of the grazing season. Table
2 shows the stocking history of the experiment. Table 3 shows how much biomass actually remained at the end of each year.
Because we had underestimated the productivity of the study site the moderate, heavy and extreme treatments were understocked
during the first two years of the study.
| Table 2. Stocking history of the grazing intensity trial. | ||||
| Year | Class of Animal | Date Stocked | Date Removed | Length of Season (days) |
| 1989 | Steers | May 22 | August 22 | 92 |
| 1990 | Bred Heifers | May 30 | November 27 | 181 |
| 1991 | Bred Heifers | May 29 | September 25 | 119 |
| 1992 | Bred Heifers | June 1 | August 25 | 85 |
| 1993 | Bred Heifers | May 29 | September 26 | 120 |
| 1994 | Open Heifers & Steers | May 17 | November 10 | 177 |
| 1995 | Open Heifers | May 18 | October 30 | 165 |
| 1996 | Open Heifers | May 20 | September 23 | 126 |
| 1997 | Open Heifers | May 27 | November 5 (August 27, Extreme)1 | 162 (92, extreme) |
| 1Livestock were removed early on the extreme treatment due to a lack of forage. | ||||
Above Ground Biomass Production
At the beginning of each grazing season five 2.7 ft2 plots were caged and two uncaged plots paired with each caged plot on each sample site. One of the uncaged plots was clipped before grazing. At the peak of biomass production two new plots were selected to match each of the original uncaged plots and the original plots were clipped. One of each pair of new plots was caged and at the end of the grazing period the herbage from each remaining plot was clipped. The herbage was separated into shrubs, forbs and grasses on overflow sites and into forbs and grasses on silty sites (occasional shrubs were placed with forbs). The samples were oven dried at 150F for 48 hours before weighing to determine the amount of herbaceous production and the percentage utilization of the forage. Herbage clipped from inside the caged plots at the peak of the growing season provides an estimate of peak biomass. The difference between the biomass in the caged plots at the end of the grazing period and the uncaged plots from the peak sampling represents the growth (or disappearance) since the peak period. The greater of peak biomass or peak biomass plus growth after peak provides an estimate of total production for the season. The data was analyzed using analysis of variance with above ground biomass yield as the dependent variable and grazing treatment as the independent variable.
A truck mounted Gettings soil probe was used to take a soil core from each sample site from as deeply as the probe would penetrate up to a maximum depth of 9 ft. The soil series was determined. The soil cores were separated into layers at 6-inch intervals for the top 3 feet and at one foot intervals below three feet. Total water, bulk density, -0.33 bar and -15 bar moisture levels were determined. Steel tubes (1 9/16" diam.) with a cap welded to the bottom to prevent water entry were installed in the holes from the soil cores and readings were taken with a neutron moisture meter. Regression analysis was used to determine the relationship between total soil water by volume and the neutron moisture meter readings. Soil water was measured around the 1st and 15th of each month when the temperature was above 32F throughout the soil profile. There were 118 sample periods between 1989 and 1997 with from 10 to 15 sample periods during each year. Total available water in each layer and accumulated total available water from each layer to the soil surface was determined. Analysis of variance was run on the data from each sample period to find significant differences between treatments
Above Ground Biomass Production
There was no significant difference in above ground biomass production among the different grazing treatments prior to 1992. Differences between treatments occurred on silty range sites in that year and they have occurred on both silty and overflow range sites in each year since. Table 4 shows total above ground biomass production by grazing treatment on the silty range sites in 1992 to 1997. In 1992 as grazing intensity increased, biomass production decreased and the heavy and extremely heavy grazing treatments produced significantly less biomass than the ungrazed and the lightly grazed treatment. Although there were no significant differences in biomass production in 1991 the fact that there were differences at the beginning of the 1992 grazing season indicates that grazing must have reduced the amount of carbohydrate reserves that the plants were able to carry over to the next season (Turner et al. 1993).
In 1993 at the beginning and mid-season sampling, above ground biomass production decreased with increased grazing intensity. The ungrazed and lightly grazed treatments produced more biomass than the other treatments and the extremely heavy treatment produced less biomass than all but the heavy treatment. In 1993, an unusually cool year, moisture was adequate, but temperature limited plant growth until August when the rains stopped. As a result the actual peak in biomass production occurred closer to the end-of-season than the mid-season sampling. By then differences in total production between treatments was not significant but grass production was still significantly less on the heavy and extremely heavy treatments than on the other treatments.
In 1994 the moderate grazing treatment produced more above ground biomass than the ungrazed and extremely heavy grazing treatments (Table 4). This was the only year that biomass production was significantly reduced on the ungrazed treatment on silty range sites compared to other grazing treatments. It is following 1993, the year with the most precipitation and the most above ground biomass production on silty range sites during the study (see Table 1). So soil water was adequate in 1994 and there was an abundance of residual vegetation on the ungrazed treatment from the previous year's production. Sharrow and Wright (1977) working in tobosagrass communities in Texas found that litter reduced yields in years when soil water was adequate and improved yields in years when soil water was limiting.
In 1995 the extremely heavy grazing treatments produced the least biomass. The other treatments were not significantly different from each other in biomass production (Table 4). In 1996 the extreme treatment again produced the least biomass and the light treatment produced the most (Table 4) and in 1997 both the extreme and heavy treatments produced less than the light treatment (Table 4).
The first year production on overflow range sites differed among treatments was 1993. Then end-of-season total yield on the
ungrazed treatment was significantly less than on all but the extremely heavy grazing treatment (Table 5). This was probably caused
by the abundant litter on the ungrazed treatment reducing the amount of sunlight reaching the surface and limiting soil temperatures.
In 1994 overflow range sites only differed between treatments in grass production at the beginning of the season. Grass growth was 698 and 688 lbs/acre on the ungrazed and extremely heavy grazing treatments, respectively, compared to 1024 to 1096 lbs/acre on the other treatments when grazing began. This was a good year for plant growth and these treatments had caught up by the mid-season sampling. In 1995 the moderate and heavy treatments produced the most biomass and the ungrazed and extremely heavy grazing treatments produced significantly less biomass than the other treatments (Table 5). Again in 1996 the ungrazed and extremely heavily grazed treatments produced significantly less biomass than the other treatments. In 1997 the moderate treatment produced the most biomass and the ungrazed and extremely heavily grazed produced significantly less. Due to their position on the landscape, overflow range sites have more available water and produce almost 40% more above ground biomass than silty range sites (see Table 1). As a result the ungrazed treatment on overflow sites has more residual vegetation than the ungrazed treatment on silty range sites. This may explain why above ground biomass on the ungrazed treatment is more consistently reduced compared to the light, moderate and heavy treatments on overflow range sites than on silty range sites.
Of the 118 times that soil water was sampled, significant differences occurred between treatments on 73 sample periods. The area was in a drought in 1988. Abundant rainfall in 1993 increased soil water levels greatly over what they were during the 1989 to 1992 period of the study. Fifty-six percent of differences occurred in the first four years of the study when soil water was more limiting. On silty range sites, the light grazing treatment may have a favorable effect on infiltration rates because the greatest increase in available water in 1993 was on that treatment.
Table 6 shows the number of times a treatment had either the most or least available water compared to the other treatments when significant differences occurred. No significant differences occurred below the 5 to 6 foot layer on overflow sites or below the 6 to 7 foot layer on silty sites.
On overflow sites the light and moderate treatments generally had more available water than the extreme treatment. The light treatment never had the least available water and the extreme treatment never had the most available water. Values for the ungrazed and heavy treatment tended to fall between those of the other three treatments. Of the periods with differences, 94% were initiated during discharge periods, that is when soil water content was lower than during the previous sample period, due to evaporation, transpiration or percolation. Six percent of the periods with differences were initiated during recharge periods, that is when soil water content was greater than during the previous sample period due to infiltration or percolation.
| Table 6. The number of times a treatment had either the most or least available water compared to other treatments when significant differences occurred (p 0.05). | ||||||||||
| Overflow Range Site | ||||||||||
| Treatment | ||||||||||
| Ungrazed | Light | Moderate | Heavy | Extreme | ||||||
| Depth(ft) | Most | Least | Most | Least | Most | Least | Most | Least | Most | Least |
| 0.0-0.51 | 3 | 1 | 9 | 0 | 0 | 1 | 1 | 3 | 0 | 8 |
| 0.0-1.0 | 3 | 2 | 14 | 0 | 3 | 0 | 1 | 6 | 0 | 13 |
| 0.0-1.5 | 3 | 2 | 10 | 0 | 5 | 0 | 1 | 1 | 0 | 16 |
| 0.0-2.0 | 1 | 1 | 3 | 0 | 14 | 0 | 0 | 0 | 0 | 17 |
| 0.0-2.5 | 0 | 1 | 2 | 0 | 10 | 0 | 0 | 1 | 0 | 10 |
| 0.0-3.0 | 0 | 1 | 2 | 0 | 9 | 0 | 0 | 1 | 0 | 9 |
| 0.0-4.0 | 0 | 0 | 1 | 0 | 5 | 1 | 0 | 1 | 0 | 4 |
| 0.0-5.0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 | 0 |
| 0.0-6.0 | 0 | 0 | 3 | 0 | 0 | 1 | 0 | 2 | 0 | 0 |
| Silty Range Site | ||||||||||
| Treatment | ||||||||||
| Ungrazed | Light | Moderate | Heavy | Extreme | ||||||
| Depth(ft) | Most | Least | Most | Least | Most | Least | Most | Least | Most | Least |
| 0.0-0.5 | 0 | 42 | 25 | 0 | 0 | 0 | 19 | 0 | 0 | 2 |
| 0.0-1.0 | 0 | 12 | 1 | 0 | 0 | 0 | 11 | 0 | 0 | 0 |
| 0.0-1.5 | 0 | 11 | 1 | 0 | 0 | 0 | 10 | 0 | 0 | 0 |
| 0.0-2.0 | 0 | 13 | 3 | 0 | 0 | 0 | 10 | 0 | 0 | 0 |
| 0.0-2.5 | 0 | 12 | 4 | 0 | 2 | 0 | 8 | 0 | 0 | 2 |
| 0.0-3.0 | 0 | 10 | 2 | 0 | 1 | 0 | 7 | 0 | 0 | 0 |
| 0.0-4.0 | 0 | 13 | 6 | 0 | 0 | 0 | 7 | 0 | 0 | 0 |
| 0.0-5.0 | 0 | 8 | 6 | 0 | 1 | 0 | 1 | 0 | 0 | 0 |
| 0.0-6.0 | 0 | 6 | 4 | 0 | 1 | 0 | 1 | 0 | 0 | 0 |
| 0.0-7.0 | 0 | 23 | 15 | 0 | 4 | 0 | 4 | 0 | 0 | 0 |
| 1Total available water in each layer from the surface down to the given depth was summed for the analysis summarized here. | ||||||||||
On silty sites the ungrazed treatment usually had the least available water. On four dates the extreme treatment had the least available water and the other treatments always had more available water than either the ungrazed or extreme treatment. Either the heavy or the light treatment usually had the most available water but the moderate treatment had the most water on about 6% of the dates when there were significant differences and the ungrazed and extreme never had the most available water. Of the periods with differences, 78% were initiated during discharge periods and 22% were initiated during recharge periods.
No differences in above ground biomass between treatments occurred prior to 1992, the fourth year of the study. This is due in part to the fact that our stocking intensities were lighter than we intended them to be during the first two years of the study and in part to the resilience of these rangelands to overgrazing. However, since 1992 on silty range sites and since 1995 on overflow range sites, this study has found a definite reduction in biomass production under extremely heavy grazing. This has been found in other studies (Maschinski and Whitham 1989, Williamson et al. 1989) and is probably explained by the amount of stress placed on the plants by grazing. This seems to be associated with reduced soil water. There is less plant cover on these sites so there is probably more runoff and evaporation from the soil surface. Rauzi (1963) in a study at the Northern Great Plains Field Station in Mandan, North Dakota found that total water intake on a moderately grazed pasture was 1.6 times greater than on a heavily grazed pasture after a one hour rainfall simulation and 1.8 times as great on an ungrazed area as on a moderately grazed pasture. The fact that there is more available water on overflow range sites compared to silty may explain why it took three more years for forage production to be reduced under extreme grazing on overflow sites than it did on silty .
There was also less production in this study on the ungrazed treatments than under moderate grazing on the silty site in 1994 and on the overflow site in 1993 and 1995 through 1997. We believe this may be a function of the amount of litter on the ungrazed treatment reducing the amount of sunlight reaching the soil surface and limiting soil temperatures. Weaver &. Rowland (1952) found that a thick mulch in Andropogon and Panicum stands reduced soil temperatures from 28 to 22F and reduced yields from 26% to 57%. Litter is more abundant on the overflow sites so it seems reasonable that it would be a limiting factor to plant growth more frequently on overflow sites than on silty sites.
The ungrazed treatment on silty sites tended to have the least available water. None of the leaf area of plants on the ungrazed treatment had been removed by grazing and so more water was probably removed by transpiration (Branson et al. 1981). The often greater production on the moderately grazed treatments may have been a result of enhanced conservation of soil moisture by reduction of the transpiration surface and reduction of mesophyll resistance relative to stomatal resistance (McNaughton 1979).
We conclude that moderate grazing may be required to maintain productivity in Northern Great Plains grasslands.
Biondini, M.E. and L. Manske. 1996. Grazing Frequency and Ecosystem Processes in a Northern Mixed Prairie, U.S.A. Ecological Applications 6:239-256.
Brand, M.D. and H. Goetz. 1986. Vegetation of Exclosures in Southwestern North Dakota. J. Range Manage. 39:434-437.
Branson, F.A., G.F. Gifford, K.G. Rehard, and R.F. Hadley. 1981. Rangeland Hydrology. Kendall/Hunt, Dubuque, Iowa, 340 pp.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas, 1402 pp.
Hart, R.H., S. Clapp and P.S. Test. 1993. Grazing Strategies, Stocking Rates, and Frequency and Intensity of Grazing on Western Wheatgrass and Blue Grama. J. Range Manage. 46:122-126.
Lauenroth, W.K., D.G. Milchunas, J.L. Dodd, R.H. Hart, R.K. Heitschmidt, and L.R. Rittenhouse. 1994. Effect of Grazing on Ecosystems of the Great Plains. pp 69-100. In: Vavra, M., W.A. Laycock, and R.D. Piper (eds). Ecological Implications of Livestock Herbivory in the West. Society For Range Management, Denver.
Maschinski, J. and T.G. Whitham. 1989. The Continuum of Plant Responses to Herbivory: The Influence of Plant Association, Nutrient Availability, and Timing. Am. Nat. 134:1-19.
McNaughton, S.J. 1979. Grazing as an Optimization Process: Grass-Ungulate Relationships in the Serengeti. Am. Nat. 113:691-703.
Rauzi, F. 1963. Water Intake and Plant Composition as Affected by Differential Grazing on Rangelands. Journal of Soil Water Conservation 18:114-116.
Sharrows, S.H. and H.A. Wright. 1977. Effects of Fire, Ash, and Litter on Soil Nitrate, Temperature, Moisture and Tobosagrass Production in the Rolling Plains. J. Range Manage. 30:226-270.
Turner, C.L., T.R. Seastedt, and M.I. Dyer. 1993. Maximization of Above-Ground Grassland Production: the Role of Defoliation Frequency, Intensity, and History. Ecological Applications 3:175-186.
Weaver, J.E. and N.W. Rowland. 1952. Effects of Excessive Mulch on Development, Yield and Structure of Native Grassland. Bot. Gaz. 114:1-19.
Williamson, S.C., J.K. Detling, J.L. Dodd, and M.I. Dyer. 1989. Experimental Evaluation of the Grazing Optimization Hypothesis. J. Range Manage. 42:149-152.
The authors are grateful to A. Nyren for her help in data analysis and manuscript preparation.
1. Bob D. Patton
Assistant Range Scientist
North Dakota State University
Central Grasslands Research Extension Center
4824 48th Ave. SE
Streeter, ND 58483
Email: bpatton@ndsuext.nodak.edu
2. Paul E. Nyren
Director/Range Scientist
North Dakota State University
Central Grasslands Research Extension Center
4824 48th Ave. SE
Streeter, ND 58483
Email: grasland@ndsuext.nodak.edu
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