Grazing Intensity Research On Coteau Rangelands

By B. Patton, P. Nyren, B. Kreft, J. Caton and A. Nyren

Contents

Summary
Introduction
Livestock Response
Forage Production And Utilization
-Treatment Effects
-Forage Utilization
-Soil Water and Forage Production
-Alternative Forage Sampling Methods
-Remote Sensing
-Forage Quality
Plant Community Dynamics

Summary

The grazing intensity study at the Central Grasslands Research Center is in its tenth year. The objectives are: to determine the optimum cattle stocking rate without damaging the rangeland resource; to develop a model to predict forage production in the spring so that livestock producers can better plan their forage requirements for the year; and to develop techniques to inventory rangeland and monitor utilization, range trend and range condition. Instruments for estimating forage production are being tested. Plant species which appear to be favored by no grazing or light grazing (decreasers), plant species which appear to be favored by moderate grazing (increaser-decreasers) and species which appear to be favored by heavy grazing (increasers) have been identified. This last group includes some species not found on ungrazed range (invaders).



Introduction

Five grazing treatments, or intensities of grazing, are included in the study: no grazing, light, moderate, heavy and extreme. Light is defined as that grazing intensity which leaves 65% of the forage produced in an "average" year at the end of the grazing season. The moderate grazing treatment is stocked to leave 50%, the heavy treatment 35% and the extreme treatment 20% of the forage produced in an average year. A certain amount of trial and error is required in adjusting stocking densities, grazing patterns and length of grazing season to achieve these grazing intensities. Each of these treatments is applied to three pastures so that differences due to grazing intensity can be separated from those due to natural variability of the pastures. Changes in the vegetation are monitored on plots located on silty and overflow range sites in each pasture. These sites are used because they are the most common in the Coteau region. Pastures with no grazing are simulated by fencing out areas on three silty range sites and three overflow range sites located within the grazed pastures.

Grazing begins each year around mid-May. Table 1 gives the stocking history of the study. To keep the same level of stress on the plants each year, grazing will continue until half of the amount of forage produced in an average year remains on the pastures grazed at the moderate rate. It will take several more years to determine the average productivity of these pastures. Table 2 gives peak total forage production for 1989 through 1998 along with the precipitation for the year. Average production for 1989 to 1998 was 3,710 lbs/acre on overflow range sites and 2,700 lbs/acre on silty range sites. Therefore, an average of 1,855 lbs/acre should remain on overflow sites and 1,350 lbs/acre on silty sites at the end of the grazing season on pastures stocked at the moderate stocking density.



Table 1. 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)

1998 Open Heifers May 16 October 28 165
1Livestock were removed early on the extreme treatment due to a lack of forage.



Table 2. Total crop year precipitation (October 1 to September 30) and peak total above ground biomass production on overflow and silty range sites on the grazing intensity study from 1989 to 1998.

Year
Precipitation
(in)
Above Ground Biomass Lbs/Acre
Overflow Silty
1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

18.40

16.10

12.89

15.25

26.59

16.86

22.60

20.55

18.63

18.91

3,863

3,847

3,142

2,758

3,999

4,201

4,773

3,837

3,351

3,334

2,089

2,962

2,629

2,065

3,446

2,803

3,134

2,645

2,376

2,855

10-Year Average 18.68 3,710 2,700



Figure 1 shows the forage remaining at the end of the grazing season for each treatment in each year of the study. Reference lines indicate the amount of forage we would like to see remaining for each grazing treatment. This shows the progress being made in adjusting stocking rates to achieve the desired use levels at the end of the grazing season.

Above ground biomass remaining (lbs/acre) on each treatment at the end of the grazing season from 1989 to 1998.
  Treatment
Year Light Moderate Heavy Extreme
1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

Ideal Remaining

2,078

2,634

2,385

1,915

2,924

2,017

2,772

2,552

2,550

2,674

2,065

2,074

2,383

1,494

1,353

2,256

1,728

1,906

1,975

1,711

1,848

1,671

2,035

2,023

833

574

1,290

1,393

1,583

1,064

689

686

984

1,701

1,985

641

406

608

901

504

513

560

522

499



Livestock Response

Table 3 shows the average daily gains and gains/acre of cattle on the trial each year from 1989 to 1998. The average body condition scores for each treatment from 1994 to 1998 are also shown on table 3. This is a visual ranking of the amount of fat on an animal's body with 10 being extremely fat and 1 being extremely thin. This ranking was made because of concern that the animals on the extreme grazing treatment were coming off in poor condition.



Table 3. Average daily gains and gains per acre from different stocking intensities 1989-1998.


Desired
Grazing
Intensity


Average Daily Gains (lbs/head/day)
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998


Light

Moderate

Heavy

Extreme

LSD(0.05)



2.18

2.35

2.03

2.00

NS2



1.01

1.23

1.17

1.05

NS



1.42a1

1.13ab

0.91b

0.69b

0.48



2.04a

1.89a

1.70a

1.20b

0.44



1.56a

1.56a

1.68a

1.06b

0.40



1.10a

0.90ab

0.74b

0.20c

0.26



1.05a

0.94a

0.86a

0.55b

0.29



1.07a

0.93a

0.81ab

0.44b

0.39



1.63a

1.46a

1.20ab

0.83b

0.45



1.53a

1.31ab

1.03b

0.60c

0.38

 

Average Gain (lbs/acre)
  1989 1990 1991 1992 1993 1994 1995 1996 1997 1998
Light

Moderate

Heavy

Extreme

LSD(0.05)

16.84c

33.27bc

41.28ab

61.00a

22.35

13.69c

27.63b

36.47b

52.87a

9.95

16.86b

43.10a

58.83a

61.90a

20.35

18.60d

54.33c

105.58b

129.22a

22.49

13.82c

45.34c

119.31b

166.77a

44.42

20.10b

38.70ab

57.23a

26.64ab

30.75

12.78c

42.37b

70.45a

77.04a

24.30

14.14c

30.10bc

53.25a

45.38ab

22.79

30.27c

66.05b

110.13a

71.10b

27.85

28.29c

62.25b

97.86a

67.98b

29.59

 

Condition Score
  1989 1990 1991 1992 1993 1994 1995 1996 1997 1998
Light

Moderate

Heavy

Extreme

LSD(0.05)

          5.19a

4.84ab

4.80ab

4.21b

0.64

5.08

5.13

5.16

4.74

NS

5.19a

5.11a

4.91ab

4.37b

0.58

5.35

5.24

4.93

-- 3

NS

5.81a

5.71ab

5.21b

4.65c

0.53

1Means in the same column followed by the same letter are not significantly different at p=0.05.
2Means not significantly different.
3Not available

Figure 2 shows the relationship between stocking rate and average daily gain, figure 3 shows the relationship between stocking rate and gain per acre and figure 4 shows the relationship between stocking rate and economic return for 1991 to 1998. Reference lines indicate the average stocking rate of each of the four grazing treatments in the study. The figures for economic return do not include the costs for land, labor or management which vary widely from one operation to another. The years 1989 and 1990 are not included in these graphs because none of the pastures were stocked heavily enough to significantly reduce average daily gains. As the grazing intensity increased, average daily gain decreased. Gains per acre increase until they reach a certain level and then begin to decline, and profit per acre shows the same pattern. As is apparent from figure 2, the relationship between stocking rate and average daily gain differs significantly between years (p<0.0005). These differences may be due to variations in forage quality, the effect of weather on the animals, class of animal, their initial weight, or their potential to gain.







 Table 4 A shows the stocking rate which would have resulted in the maximum gain per acre in each year. We can't predict what stocking will result in the maximum gain in the future so its impossible to stock each year for the maximum gain in that year. In retrospect if we were to pick one constant stocking rate for the past eight years which would have provided the greatest gain per acre for this period it would have been 2.385 AUMs/acre. This is the point labeled as "optimum" in figure 3. Table 4 B shows what the gain/acre would have been in each year if we had stocked at this rate. Table 4 C shows what the gain per acre would have been if we had stocked at a moderate stocking rate. It is important to note that the pastures which are stocked at the heavy and extreme rates were in good condition at the beginning of the study and that their condition has slowly deteriorated over the course of the study. As these pastures have deteriorated we have seen the "optimum" stocking rate gradually decline and we expect that we will see it decline further as the study continues.


Table 4. Comparison of gain per acre from selected stocking rates.
A B C
Stocking rate that would result in the maximum gain/acre in each year. Stocking rate that if held constant would result in the maximum gain/acre over the eight-year period. Gain/acre over the eight-year period if stocking rate were held constant at 0.95 AUMs/acre which was the average of the moderate treatment over this period.
Year aums
/acre
gain
/acre
aums
/acre
gain
/acre
aums
/acre
gain
/acre
1991 2.258 62.526 2.385 62.324 0.953 41.168
1992 2.681 134.753 2.385 133.073 0.953 77.294
1993 3.406 175.776 2.385 158.226 0.953 74.510
1994 2.268 58.117 2.385 57.959 0.953 38.077
1995 3.076 84.690 2.385 80.201 0.953 42.325
1996 2.036 56.957 2.385 55.064 0.953 38.711
1997 1.919 92.431 2.385 86.127 0.953 65.335
1998 2.082 91.219 2.385 88.954 0.953 59.881
8 yr.ave. 2.466 94.559 2.385 90.241 0.953 54.663



If cattle prices were constant, return per acre would peak at a stocking rate somewhere below maximum gain per acre with the exact point depending on carrying costs (interest, death loss, salt and mineral, vet cost, transportation, labor and land). However, fluctuating cattle prices make determining an optimum stocking rate difficult. For example, in May 1991 we stocked our pastures with bred heifers weighing an average of 800 lbs. An 800 lb heifer was valued at $702.26. Our carrying cost for the season for this animal was $35.29, so if we had sold her in September when the cattle were removed from the trial we would have had to get $737.55 to break even. To get this price, assuming a 5% shrink, she would have had to come off the pasture weighing 941 lbs which would be an average daily gain of 1.18 lbs/day. Average daily gains were not very good in 1991 therefore reducing economic returns. In May 1992, we stocked our pastures with bred heifers averaging 750 lbs. Heifers of this weight were valued at $560.33 in May and at $620.33 in August at the end of the grazing season. Carrying costs for this animal in 1992 were approximately $19.77 which would have returned $40.13 even without a gain in weight. This difference in beginning and ending value of the animals made it possible for the stocking rate with maximum return/acre to exceed the stocking rate with maximum gain/acre in 1992.

Table 5 A gives the stocking rates with the maximum predicted return per acre for each year from 1991 to 1998. These values correspond to the peaks of the curves in figure 4. Just as we can't predict what stocking will result in maximum gain per acre we can't predict what stocking rate will provide the greatest economic return in any future year. With cattle prices of the last eight years, and the gains that we achieved on these pastures, the one constant stocking rate which would have given us the greatest economic return over the last eight years was 1.739 AUMs/acre. This is the point labeled optimum in figure 4. Table 5 B shows what the return/acre would have been if we had stocked at this rate. Table 5 C shows what the return would have been if we had stocked at a moderate rate. Although the average return per acre is higher under the optimum rate there were three years with negative returns while all years had positive returns under the moderate stocking rate. (Cost for land, labor and management have not been subtracted.) Comparing tables 4 and 5 it can be seen that in all but two years, 1992 and 1996, the stocking rate with the greatest economic return was less than the rate with the greatest gain per acre. Again it must be stressed that the pastures stocked at the heavy and extreme rates were in good condition at the beginning of the study. As these pastures have deteriorated in condition the "optimum" stocking rate has gradually declined. As the study continues we expect that we will see further reductions in livestock gains on these heavily grazed pastures as they continue to deteriorate, which should have the effect of lowering the stocking rate with the "optimum" return.


Table 5. Comparison of return to land, labor and management from selected stocking rates.
A B C
Stocking rate that would result in the maximum return/acre to land, labor and management in each year. Stocking rate that if held constant would result in the maximum return to land, labor and management over the eight-year period. Returns/acre to land, labor and management over the eight-year period if stocking rate were held constant at 0.95 aums/acre which was the average of the moderate treatment over this period.
Year aums
/acre
returns
/acre
gain
/acre
aums
/acre
returns
/acre
gain
/acre
aums
/acre
returns
/acre
gain
acre
1991

1992

1993

1994

1995

1996

1997

1998

8 yr. ave.

0.882

3.125

2.387

0.669

1.439

2.060

1.113

1.012

1.586

$4.10

$97.11

$105.10

$1.99

$2.05

$31.83

$13.35

$2.11

$32.20

38.764

130.956

158.275

28.489

59.491

56.948

73.542

63.066

76.191

1.739

1.739

1.739

1.739

1.739

1.739

1.739

1.739

1.739

-$1.35

$77.31

$97.16

-$4.52

$1.44

$30.96

$8.62

-$3.17

$25.81

59.151

117.696

129.017

54.876

67.894

55.586

91.493

88.336

83.006

0.953

0.953

0.953

0.953

0.953

0.953

0.953

0.953

0.953

$4.06

$48.47

$66.13

$1.53

$0.46

$21.51

$13.05

$2.07

$19.66

41.168

77.294

74.510

38.077

42.325

38.711

65.335

59.881

54.663



This trial is continuing and these relationships need more study before we can recommend the stocking rate which will give the greatest return in the long run. Stocking at the rate which produced the greatest economic return in 1992 would damage the pasture and, if prices are low, it could result in substantial financial losses. The returns from a moderate stocking rate are "moderate," higher than the "optimum" stocking rate when prices are poor, and lower when prices are good. We cannot yet recommend the 1.739 AUMs/acre in table 5 because it is only based on eight years of data and changes in forage production, particularly as a result of range deterioration, or cattle prices could make it inappropriate. Also producers should keep in mind that an optimum stocking rate for a stocker operation may not be optimum for a cow-calf operation. In the future we hope to provide more information to help the livestock producer make a sound decision.



 Forage Production And Utilization

Forage production is determined by clipping plots inside of wire cages which exclude grazing, and forage remaining is determined by clipping paired plots located outside of the cages. The amount of forage used by livestock is determined by comparing the two. This sampling is very time consuming and labor intensive, but it provides information which can be obtained in no other way.

Treatment Effects

We can determine if the grazing treatments affect forage 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 6 shows total above ground biomass production by grazing treatment on the silty range sites in 1992 to 1998. 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).

 

Table 6. Above ground biomass production by grazing treatment on silty range sites from 1992 to 1998.
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season


Above ground biomass (lbs/acre) 1992
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1993
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1994
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1995
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1996
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1997
None

Light

Moderate

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

Above ground biomass (lbs/acre) 1998
None

Light

Moderate

Heavy

Extreme

LSD(0.05)

1,918 a

1,796 a

1,966 a

1,119 b

1,304 b

360

2,752 a

2,883 a

2,667 ab

2,136 b

1,418 c

555

3,425

3,091

3,387

2,383

1,990

NS

3,425

3,025

3,387

2,363

1,990

NS

1Means in the same column followed by the same letter are not significantly different at P=0.05.
2Means not significantly different.

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 6). 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 2). 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 6). In 1996 the extreme treatment again produced the least biomass and the light treatment produced the most (Table 6) and in 1997 both the extreme and heavy treatments produced less than the light treatment (Table 6). In 1998 both the heavy and extreme treatments had less production than the other treatments at the beginning of the grazing season and they still had less production than the ungrazed and light at mid season but treatments did not differ significantly beyond that point (Table 6).

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 7). This was probably caused by the abundant litter on the ungrazed treatment reducing the amount of sunlight reaching the surface and limiting soil temperatures.


Table 7. Above ground biomass production by grazing treatment on overflow range sites from 1993 to 1998.
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season

Above ground biomass (lbs/acre) 1993
None

Light

Moderate

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


Above ground biomass (lbs/acre) 1994
None

Light

Moderate

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


Above ground biomass (lbs/acre) 1995
None

Light

Moderate

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


Above ground biomass (lbs/acre) 1996
None

Light

Moderate

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


Above ground biomass (lbs/acre) 1997
None

Light

Moderate

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


Above ground biomass (lbs/acre) 1998
None

Light

Moderate

Heavy

Extreme

LSD(0.05)

1,460

1,393

1,324

1,432

1,381

NS

2,767

3,638

3,315

3,063

2,186

NS

2,883

3,660

4,003

3,420

2,703

NS

2,578

3,532

4,003

3,376

2,613

NS

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 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 7). 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. In 1998 there were not significant differences in total production between treatments but there were differences in the amount of forbs and in growth rates between treatments. At the beginning of the grazing season the extreme treatment had the most forbs and the ungrazed treatment had the least. But the extreme treatment had the least growth in forbs between the beginning of grazing and the mid-season sampling. The light treatment had the greatest total growth during this period and the extreme treatment had the least. The rate of growth between the time the cattle were put on pasture and the last forage sampling period was greatest on moderate and least on ungrazed. 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 2). 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.

Forage Utilization

Forage utilization data are used to help place mineral blocks and adjust livestock numbers to get the desired grazing intensity on the sample sites in each pasture. Table 8 gives the estimated percent forage disappearance at the time the cattle were removed. The term disappearance is used because forage lost due to the natural drying up and breaking off of leaves and stems is included with that consumed or trampled by cattle. Several facts are apparent from examination of this table. First, a substantial amount of the forage produced had disappeared even on the overflow sites on the ungrazed treatment. The ungrazed silty treatment has a negative value for utilization because there was more forage at the end than at the middle of the growing season indicating that growth exceeded disappearance. Second, percent disappearance was less than ideal disappearance for all but the heavy grazing treatment. Some adjustments will be made I the number of livestock and the location of salt and mineral blocks in some of the pastures in 1999 to bring the grazing treatments closer to the desired levels.



Table 8. Estimated percent forage disappearance when the cattle were removed from the grazing intensity trial in 1998.
 
Percent Disappearance1




Treatment


Overflow


Silty
Average
For
Treatment
Ideal
Disappearance
for Pasture2
None 11 -3 -- --
Light 33 9 12 33
Moderate 46 51 50 55
Heavy 61 76 74 61
Extreme 78 76 77 78



2Percent disappearance required to achieve the desired grazing intensity on each treatment, adjusted for current year's production.



Soil Water and Forage Production

Forage production will be correlated with soil moisture and precipitation to develop a model to predict forage production. Data will have to be collected for a number of years before a model can be developed. However, we are seeing differences in available water between the different grazing treatments. On overflow range sites, lightly grazed pastures have more available water than heavily grazed pastures. The differences in available water occur during both soil water recharge and discharge. This indicates that on heavily grazed sites more water runs off during a rain and sunlight evaporates more water from the soil surface. On silty range sites, moderately grazed pastures have more available water than ungrazed or heavily grazed pastures. The ungrazed treatment has less available water because the plants on that treatment have more leaf area than the grazed plants, and more water is removed from the soil by transpiration.

Alternative Forage Sampling Methods

The forage production samples are used to calibrate and test the swardstick and radiometer, two alternative methodologies for sampling forage production (see Using Remote Sensing to Manage North Dakota's Rangelands).

Remote Sensing

Forage production from known locations will be compared with reflectance values on infrared and regular color aerial photos. The photos can be scanned into a computer and analyzed to develop a map and comprehensive inventory of the entire forage base (see Using Remote Sensing to Manage North Dakota's Rangelands).

Forage Quality

The forage samples will be analyzed each year for nutritional quality to determine if, over time, different intensities of grazing result in plant communities which produce forage of different quality. Table 9 shows the average nutritional quality of grasses and forbs on each treatment from 1989 to 1998. Although differences in nutritional quality are developing between the grazing treatments, the reasons for the differences are not clear. On silty range sites the grasses have higher crude protein and digestibility and lower fiber components at the higher grazing intensities. On the heavily grazed treatments the grass that is available for grazing is mostly regrowth which is of higher quality. However on overflow sites both grasses and forbs are highest in fiber components on the heavy grazing treatment. Perhaps on these sites cattle are selecting species of higher quality and leaving those that are higher in fiber. On silty sites forbs are highest in neutral detergent fiber on the ungrazed and extreme grazing treatments. As the ungrazed forage matures on the ungrazed treatment it becomes higher in fiber. On the heavily grazed treatments only forbs of lower quality would remain ungrazed. These differences in nutritional quality have occurred gradually over the course of the study.



Table 9. Average nutritional quality of forage on the grazing intensity trial 1989-1998.
Treatment Crude
Protein
(%)
In Vitro
Dry Matter
Digestibility

(%)
Acid
Detergent
Lignin
(%)
Acid
Detergent
Fiber
(%)
Neutral
Detergent
Fiber
(%)

Overflow sites--forbs

None
Light
Moderate
Heavy
Extreme
9.17 c1
8.80 c
9.13 c
10.42 a
9.87 b
61.86
60.53
61.74
59.99
60.14
6.80 c
7.52 ab
7.31 abc
7.81 a
7.23 bc
35.98
38.08
37.67
37.19
36.82
43.06 b
44.85 ab
44.79 ab
46.40 a
44.64 ab
Overflow sites--grasses
None
Light
Moderate
Heavy
Extreme
6.57 d
7.03 bc
6.73 cd
7.29 ab
7.57 a
50.65
51.38
50.63
49.91
52.12
4.67
4.42
4.58
4.61
4.57
42.56 b
42.80 ab
43.02 ab
43.85 a
41.01 c
67.03 c
67.69 bc
68.46 ab
69.37 a
68.85 ab
Silty sites-forbs
None
Light
Moderate
Heavy
Extreme
10.40
10.75
10.88
10.79
10.76
59.22 b
61.94 a
60.48 ab
60.33 ab
62.00 a
7.80
7.35
7.41
7.73
7.31
36.58 a
36.15 ab
34.43 c
34.97 bc
33.86 c
50.01 a
45.40 b
44.85 b
45.42 b
48.70 a
Silty sites-grasses
None
Light
Moderate
Heavy
Extreme
7.35 c
7.36 c
7.88 b
8.40 a
8.55 a
49.11 c
46.68 d
50.55 bc
51.14 b
55.86 a
4.17 b
4.64 a
4.19 b
4.24 b
4.04 b
42.84 ab
43.64 a
42.32 b
40.83 c
39.31 d
69.44 bc
72.05 a
71.43 ab
71.48 ab
68.37 c
1Means in the same column followed by the same letter, or no letter, are not significantly different at P=0.05.




Plant Community Dynamics

Changes in the plant communities are monitored by sampling the percent frequency of occurrence, density per unit area, and percent basal cover of all plant species as well as sampling the weight of herbage produced. Frequency of occurrence is sampled by placing a frame on the ground fifty times along a transect at each sampling site. Every time it is placed on the ground all the plant species which occur in the frame are recorded. The number of frames a species occurs in, divided by the total number of frames, multiplied by 100, is the percent frequency of that species. The frequency value obtained from sampling is dependent on the size of the frame. If the frame is too small, the species will rarely be recorded and if the frame is too large, the species will be present in almost every frame. Because species may differ in their abundance on a particular site, and a particular species may differ in its abundance on different sites, it is not possible to select a single frame size that is optimum for all species on all sites. For this study, a 25 x 25 cm frame with 5 x 5 cm and 10 x 10 cm frames nested within it, is being used.

Density is determined by counting all the individual plants of each species in each of the frames. Density data were only collected on shrubs and forbs from 1988 to 1991. In 1992 we began collecting density data on cespitose (bunch) grasses and sedges. Density data are not collected on rhizomatous grasses and grass-like plants because of the difficulty of counting individual plants.

Basal cover is sampled by repeatedly placing a 10 point frame on the ground and recording each time one of the points strikes the base of a plant. The number of times a point hits the base of a plant divided by the number of points read, times 100, is the percent basal cover of that species. Basal cover is stable from year to year because it doesn't change unless old plants die or new plants are established. Also the point frame can be used to sample the amount of litter or bare ground as well as total plant basal cover. However, it requires too large a sample size to provide reliable data for all but the dominant species of a plant community. The point frame was used to sample litter and bare ground in 1996. Frequency can be sampled more quickly than density or basal cover which makes it an ideal method for monitoring range trend or the direction of change in range condition. These three methods of monitoring vegetation change can be complementary and are used together in this study. Frequency data from 1988 to 1998, density data from 1988 and 1990 to 1998 and basal cover data from 1988, 1990, 1992, 1993 and 1996 were compared for each sample site using analysis of variance. The change in abundance of species between years was determined for each site. The arcsine transformation was applied to frequency and basal cover data to convert it from a binomial distribution to a nearly normal distribution. Analysis of variance was performed to determine if there was a change in species abundance across all sites which might indicate a response to weather, or if there was a change in response to the different grazing treatments. All tests were performed at the p=0.05 level. Table 10 lists those species which appear to be favored by no grazing or light grazing. Table 11 lists those species which appear to be favored by moderate grazing, and table 12 lists those species which appear to be favored by heavy grazing. Table 12 also indicates species which appear to be invasive. These are species which have not been found or are extremely rare on the ungrazed or lightly grazed treatments. In addition to the changes listed for plant species, litter has decreased on overflow range sites and bare ground has increased on both silty and overflow range sites under heavy grazing.

 
Table 10. Plant species which appear to be favored by no grazing or light grazing (P < 0.05.)
Overflow Range Sites
Galium boreale
Helianthus rigidus
Rosa arkansana
Tragopogon dubius
northern bedstraw
stiff sunflower
prairie rose
goat's beard
Silty Range Sites
Ambrosia psilostachya
Artemisia absinthium
Helianthus rigidus
Melilotus officinalis
Poa pratensis
Psoralea esculenta
Stipa comata
Tragopogon dubius
western ragweed
wormwood
stiff sunflower
yellow sweetclover
Kentucky bluegrass
breadroot scurf-pea
needle-and-thread
goat's beard
 
 
 


Table 11. Plant species which appear to be favored by moderate grazing (P< 0.05.)
Overflow Range Sites
Ambrosia psilostachya
Anemone canadensis
Anemone cylindrica
Campanula rotundifolia
Poa pratensis
Sisyrinchium montanum
Solidago rigida
western ragweed
meadow anemone
candle anemone
harebell
Kentucky bluegrass
blue-eyed grass
stiff goldenrod
Silty Range Sites
Agropyron repens
Artemisia ludoviciana
Dichanthelium wilcoxianum
Psoralea argophylla
Ratibida columnifera
Sisyrinchium montanum
Stipa curtiseta
quackgrass
cudweed sagewort
Wilcox dichanthelium
silver-leaf scurf-pea
prairie coneflower
blue-eyed grass
needlegrass

 

Table 12. Plant species which appear to be favored by heavy grazing (P< 0.05).

Overflow Range Sites
Achillea millefolium
Agropyron caninum
Agropyron smithii
Agrostis hyemalis
Androsace occidentalis
Astragalus agrestis
Aster ericoides
Carex heliophila
Cerastium arvense
Cirsium flodmanii
Conyza canadensis
Erysimum inconspicuum
Grindelia squarrosa
Juncus interior
* Medicago lupulina
Oxalis stricta
* Polygonum ramosissimum
Potentilla norvegica
* Potentilla pensylvanica
Solidago missouriensis
Stipa viridula
Taraxacum officinale
Viola pedatifida
western yarrow
slender wheatgrass
western wheatgrass
ticklegrass
western rock jasmine
field milk-vetch
heath aster
sun sedge
prairie chickweed
Flodman's thistle
horse-weed
smallflower wallflower
curly-cup gumweed
inland rush
black medic
yellow wood sorrel
bushy knotweed
Norwegian cinquefoil
Pennsylvania cinquefoil
Missouri goldenrod
green needlegrass
common dandelion
larkspur violet

Silty Range Sites
Achillea millefolium
* Agrostis hyemalis
Androsace occidentalis
Artemisia frigida
Aster ericoides
Carex eleocharis
Carex heliophila
Cerastium arvense
Chenopodium desiccatum
Draba nemorosa
Erysimum asperum
Euphorbia serpyllifolia
Grindelia squarrosa
Hedeoma hispidum
Lepidium densiflorum
Oxalis stricta
Plantago patagonica
* Polygonum ramosissimum
Potentilla pensylvanica
Stipa viridula
Symphoricarpos occidentalis
western yarrow
ticklegrass
western rock jasmine
fringed sagewort
heath aster
needle-leaved sedge
sun sedge
prairie chickweed
narrow-leaved goosefoot
yellow whitlowort
western wallflower
thyme-leaved spurge
curly-cup gumweed
rough false pennyroyal
peppergrass
yellow wood sorrel
wooly plantain
bushy knotweed
Pennsylvania cinquefoil
green needlegrass
buckbrush
* Invaders, these species have not been found, or are extremely rare, on the ungrazed and lightly grazed treatments.
 

The value of the plant community for grazing depends on the plant species present and their forage quality. As livestock select plants of high palatability in their diet, they give a competitive advantage to plants of lower palatability and cause a shift in the composition of the plant community. An ungrazed pasture may consist of high quality forage but it is of no use to a livestock producer unless the cattle are allowed to graze it. Likewise, a pasture which has been continually overgrazed may consist of plants of low forage quality and would be of little use to a livestock producer. This research will continue to monitor the changes in the vegetation and livestock performance for at least two more years.

Bob Patton
Assistant Range Scientist
North Dakota State University
Central Grasslands Research Center
4824 48th Ave. SE
Streeter, ND 58483
Phone: 701-424-3606
E-mail: bpatton@ndsuext.nodak.edu

Previous article *Return to Contents *Next article