North Dakota State University
North Dakota Agricultural Experiment Station
Central Grasslands Research Center


The Effect of Grazing Intensity on Soil Water and Rangeland Productivity in South-Central North Dakota

Bob D. Patton and Paul E. Nyren

ABSTRACT: 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.

KEY TERMS: Grazing pressure; available soil water; biomass production; stocking rate; forage production; grazing management.



INTRODUCTION

The Great Plains grasslands evolved under grazing by large herbivores. 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? 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.



MATERIALS AND METHODS

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 forage production each year of the study.


Table 1. Total crop year precipitation (October 1 to September 30) and peak total forage production on overflow and silty range sites on the grazing intensity study from 1989 to 1997.
Year Precipitation

(in)

Lbs/Acre
Overflow Silty
1989 18.40 3,863 2,089
1990 16.10 3,847 2,962
1991 12.89 3,142 2,629
1992 15.25 2,720 2,067
1993 26.59 3,999 3,446
1994 16.86 4,656 3,412
1995 22.60 4,773 3,134
1996 20.55 3,837 2,645
1997 18.63 3,368 2,459
9-Year Average 18.65 3,754 2,692

 

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.


Experimental Design

The experiment, which began in 1989, is organized 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 forage production. This required varying the length of the grazing season. Table 2 shows the stocking history of the experiment. Table 3 shows how much forage actually remained at the end of each year.


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 86
1993 Bred Heifers May 29 September 26 118
1994 Open Heifers & Steers May 17 November 10 178
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
176
(106, extreme)
1Livestock were removed early on the extreme treatment due to a lack of forage.



Table 3. Forage remaining (lbs/acre) on each treatment at the end of the grazing season from 1989 to 1997.
  Treatment
Year Light Normal Heavy Extreme
1989 2,078 2,074 2,035 1,701
1990 2,634 2,383 2,023 1,985
1991 2,385 1,494 833 641
1992 1,915 1,353 574 406
1993 2,924 2,256 1,290 608
1994 2,017 1,728 1,393 901
1995 2,772 1,906 1,583 504
1996 2,552 1,975 1,064 513
1997 2,683 1,728 890 530
Ideal Remaining 2,083 1,652 992 505


Forage Production

At the beginning of each grazing season five 2.69 sq. foot 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 picked 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 forage yield as the dependent variable and grazing treatment as the independent variable.


Soil Water

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.



RESULTS AND DISCUSSION

Forage Production

There was no significant difference in forage 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 forage production by grazing treatment on the silty range sites in 1992 to 1997. In 1992 as grazing intensity increased, peak forage production decreased and the heavy and extremely heavy grazing treatments produced significantly less forage than the ungrazed and the lightly grazed treatment. In 1993 at the mid-season sampling, forage production decreased with increased grazing intensity. The ungrazed and lightly grazed treatments produced more forage than the other treatments and the extremely heavy treatment produced less forage than all but the heavy treatment.

1993 was an unusually cool year, and although moisture was adequate, temperature limited plant growth until August when the rains stopped. As a result the actual peak in forage 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 normal grazing treatment produced the most forage. Production on this treatment was significantly greater than on both the ungrazed and extremely heavy grazing treatments (Table 4). This may indicate that in the long-term both non-use and overuse can be detrimental to the productivity of native rangeland. In 1995 the extremely heavy grazing treatments produced the least forage. The other treatments were not significantly different from each other in forage production (Table 4). In 1996 the extreme treatment again produced the least forage 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).


Table 4. Forage production by grazing treatment on silty range sites from 1993 to 1997.
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season
(lbs/acre)

1992

None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,899 a1
1,534 ab
1,398 b
895 c
648 c
388
2,490 a
2,320 ab
2,133 ab
1,668 bc
1,424 c
712
2,608 a
2,438 a
2,149 ab
1,668 b
1,462 b
742
2,208
2,219
1,715
1,432
1,421
NS2
1993
None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,680a
1,361b
997c
763d
667d
195
2,391 a
2,561 a
2,127 b
2,028 bc
1,847 c
232
3,467
4,024
3,723
2,835
3,180
NS
3,467
4,024
3,723
2,835
3,180
NS
1994
None
Light
Normal
Heavy
Extreme
LSD(0.05)
939
1,342
1,365
1,102
669
NS
1,999 b
2,810 a
2,867 a
2,493 ab
2,084 b
591
2,245 c
2,898 abc
3,416 a
3,015 ab
2,442 bc
767
2,106 c
2,621 abc
3,416 a
3,015 ab
2,442 bc
795
1995
None
Light
Normal
Heavy
Extreme
LSD(0.05)
805 a
881 a
605 ab
512 b
365 b
281
3,129 a
3,704 a
3,274 a
3,001 ab
2,244 b
764
3,270 a
3,704 a
3,274 a
3,001 ab
2,421 b
721
2,521
2,698
2,993
2,481
2,390
NS
1996
None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,175
1,675
1,238
1,070
444
NS
2,737 a
2,920 a
2,843 a
2,256 ab
1,513 b
784
2,883 ab
3,357 a
3,071 ab
2,282 bc
1,631 c
926
2,632
3,227
2,913
2,078
1,617
NS
1997
None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,482 a
1,450 a
1,378 a
928 a
749 b
378
2,341 ab
2,660 a
2,491 ab
2,110 b
1,345 c
497
2,651 ab
3,217 a
2,617 ab
2,369 b
1,444 c
782
2,530 a
3,217 a
2,573 a
2,369 a
1,444 b
870
1Means in the same column followed by the same letter are not significantly different at P=0.05.
2Means not significantly different.


Production on overflow range sites only differed among treatments in end-of-season total yield in 1993 (Table 5). Here the ungrazed treatment produced significantly less forage than all but the extremely heavy grazing treatment. This was probably caused by the abundant litter on the ungrazed treatment reducing the amount of sunlight reaching the surface and limiting soil temperatures. 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.


Table 5. Forage production by grazing treatment on overflow range sites from 1993 to 1997.
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season
(lbs/acre)

1993

None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,422
1,454
1,459
1,503
1,102
NS2
2,810
3,164
3,161
4,112
2,547
NS
2,949
4,075
4,253
4,920
3,798
NS
2,665 b1
4,017 a
4,253 a
4,920 a
3,798 ab
1,310
1994
None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,054
1,161
1,284
1,280
833
NS
3,448
4,586
4,630
4,920
2,727
NS
3,448
4,686
4,742
5,013
3,118
NS
2,685
3,997
4,555
4,874
3,118
NS
1995
None
Light
Normal
Heavy
Extreme
LSD(0.05)
614 cd
709 bc
958 ab
1,083 a
339 d
333
4,060 ab
5,255 a
5,358 a
5,399 a
2,992 b
1,519
4,060 b
5,255 a
5,777 a
5,540 a
3,235 b
1,192
2,848 c
4,216 b
4,916 ab
5,540 a
2,762 c
1,144
1996
None
Light
Normal
Heavy
Extreme
LSD(0.05)
964 b
1,033 b
1,374 a
1,507 a
633 c
303
3,583 a
4,236 a
4,090 a
4,347 a
2,343 b
1,184
3,583 a
4,236 a
4,235 a
4,522 a
2,608 b
974
2,457 b
3,836 a
4,004 a
4,395 a
2,608 b
1,065
1997
None
Light
Normal
Heavy
Extreme
LSD(0.05)
1,192
1,040
1,110
1,421
1,032
NS
2,849 ab
3,831 a
3,643 a
3,399 a
1,830 b
1,215
2,863 bc
3,831 ab
4,299 a
3,937 ab
1,909 c
1,420
2,747 bc
3,365 ab
4,025 a
3,937 ab
1,782 c
1,530
1Means in the same column followed by the same letter are not significantly different at P=0.05.
2Means not significantly different.


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 normal and heavy treatments produced the most forage and the ungrazed and extremely heavy grazing treatments produced significantly less forage than the other treatments (Table 5). Again in 1996 the ungrazed and extremely heavily grazed treatments produced significantly less forage than the other treatments. In 1997 the normal treatment produced the most forage and the ungrazed and extremely heavily grazed produced significantly less.

Soil Water

Of the 118 times that soil water was sampled, significant differences occurred between treatments on 73 sample periods. The area suffered from 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 normal 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. Ninety-four percent of the periods with differences 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.

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 normal 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. Seventy-eight percent of the periods with differences were initiated during discharge periods and 22% were initiated during recharge periods.



CONCLUSIONS

This study found a definite reduction in biomass production on both overflow and silty range sites 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.

There was also less production in this study on the ungrazed treatments than under moderate grazing. This trend was stronger on overflow range sites than it was on silty range sites, and it was stronger in some years than others. 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.

The ungrazed treatment on silty sites tended to have the least available water. Although production was often greater on the moderately grazed treatments, 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).

We conclude that moderate grazing may be required to maintain productivity in Northern Great Plains grasslands.


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 Normal Heavy Extreme
Depth(ft) Most Least Most Least Most Least Most Least Most Least
0.0-0.5 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 Normal 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



Acknowledgements

The authors are grateful to A. Nyren for her help in data analysis and manuscript preparation.



References

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.

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.

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.

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.


March 1998


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North Dakota State University
North Dakota Agricultural Experiment Station
Central Grasslands Research Center