Grazing Intensity Research in the Missouri Coteau of North Dakota



Livestock Response


        Table 3 shows the average daily gain, gain per acre and body condition scores from the different grazing intensities for the last five years, average gains by treatment from 1991 to 2002 and average body condition from 1994 to 2002. Grazing pressure was too light on the heavy and extreme treatments in the first two years of the study so there are no significant differences in average daily gains in 1989 and 1990. Following that year average daily gain and animal body condition scores decrease with increasing grazing intensity. The rate at which average daily gain decreases with an increase in stocking rate varies greatly from year to year. The differences between years may be due to variation in forage quality or quantity, the effect of weather on the animals, their initial weight, or their potential to gain.


        The past two years were unusual. In 2001 precipitation was well above average in June and July and well below average in August and September. This reduced the quantity of forage produced but in these situations the forage often cures at a higher quality. 2002 was very dry in May and June and forage production was the least of any year (table 2). The lack of forage forced us to remove the cattle on September 11, 2001 and on July 17, 2002, the earliest they had come off since 1992 (see table 1). The theory that forage quality was higher than average is supported by the fact that there was no significant difference in animal body condition scores between treatments, the first time this has occurred since 1995 (condition scores for cattle on the extreme treatment were not available in 1997). It was also observed that the quality of animals stocked on the study in 2001 was better than usual. They were not as wild and they fed better. The cattle probably gain well on all treatments at the beginning of the grazing season and reduce their rate of gain on the heavy treatments as forage quantity becomes limiting. In years when the grazing season ends early as in 2000 to 2002 there is less chance for the differences in rate of gain between the light and extreme treatments to become significant. If all years are pooled together the relationship between rate of gain and stocking rate has an R2 of 0.40, which means that stocking rate explains 40% of the variation in average daily gain between pastures. When only one year is considered at a time the R2 varies between 0.45 and 0.98, with the 0.45 coming from 2001. The next lowest was 2002 with an R2 of 0.48, and third was 1993 with an R2 of 0.59. Therefore ignoring 2001 and 2002, between 59% and 98% of variation in average daily gain is a result of the difference in stocking rate. The relationships between stocking rate and average daily gain are illustrated in figure 2. Reference lines indicate the average stocking rates for each of the four grazing treatments.



Table 3. Average daily gains, gains per acre, and condition scores from different stocking intensities.

 

Desired

Grazing

Intensity

Average Daily Gains (lbs/head/day)

1998 1999 2000 2001 2002 Average

1991-2002

Light 1.53a1 1.40a 1.12 1.44 1.34 1.39a
Moderate 1.31ab 1.30a 0.967 1.29 1.47 1.27a
Heavy 0.963b 1.19ab 0.97 1.23 0.960 1.11b
Extreme 0.60c 0.96b 0.82 1.14 0.78 0.77c
LSD(0.05) 0.38 0.25 NS2 NS NS 0.16
             
  Average Gain (lbs/acre)
Desired

Grazing

Intensity

1998 1999 2000 2001 2002 Average

1991-2002

Light 28.29c   36.50b 33.03c  43.18c 20.06 24.39c
Moderate 62.25b   59.73b 42.39bc  59.88bc 37.90 48.51b
Heavy 97.86a   93.93a 58.24ab  67.15b 33.57 77.13a
Extreme 67.98b 108.49a 74.44a 108.27a 38.96 81.35a
LSD(0.05) 29.59   24.31 17.52  23.74 NS 12.73
             
  Condition Score
Desired

Grazing

Intensity

1998 1999 2000 2001 2002 Average

1994-2002

Light 5.81a 5.72a 5.18a 5.78 5.22 5.39a
Moderate 5.71ab 5.65ab 5.20a 5.52 5.18 5.29ab
Heavy 5.21b 5.54bc 5.01a 5.43 5.18 5.13b
Extreme 4.65c 5.41c 4.61b 5.24 5.05 4.78c
LSD(0.05) 0.53 0.18 0.31 NS NS 0.21
             
1Means in the same column followed by the same letter are not significantly different at p=0.05.

2Means not significantly different.


Initially gain/acre increases as the stocking rate increases but there comes a point when further increases in stocking rates result in reduced gain/acre (see figure 3). What happens at stocking rates beyond the extreme stocking rate reference line is mainly hypothetical because we have very few observations on which to base our regression lines. However all but 2001 had at least one observation from a stocking rate higher than the projected rate which would provide maximum gain/acre for the year. Table 4 shows the stocking rate that would have resulted in the maximum gain/acre in each year. Since we can’t predict ahead of time what stocking rate would give the maximum gain/acre in a particular year it would be impossible to stock each year for maximum gain/acre. In retrospect, if we were to pick one stocking rate that would have resulted in the maximum gain/acre over this 12-year period it would have been 2.14 AUM/acre. This is the point labeled optimum in figure 3.

 

Table 4. Comparison of gain in lbs 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 12-year period.

Gain/acre over the 12-year period if stocking rate were held constant at 0.96 AUMs/acre, the average of the moderate treatment over this period.

Year

AUMs

/acre

gain

/acre

AUMs

/acre

gain

/acre

AUMs

/acre

gain

/acre

1991

2.26

62.5

2.14

62.4

0.96

41.3

1992

2.68

134.8

2.14

129.2

0.96

77.6

1993

3.41

175.8

2.14

148.9

0.96

74.9

1994

2.27

58.1

2.14

57.9

0.96

38.2

1995

3.08

84.7

2.14

76.5

0.96

42.5

1996

2.04

57.0

2.14

56.8

0.96

38.9

1997

1.92

92.4

2.14

90.96

0.96

65.6

1998

2.08

91.2

2.14

91.1

0.96

60.1

1999

3.18

111.4

2.14

98.4

0.96

51.76

2000

2.81

76.6

2.14

72.2

0.96

42.7

2001

 

*

2.14

99.9

0.96

50.8

2002

0.94

41.5

2.14

-44.6

 

41.5

12-year Avg.

2.42

89.6

2.14

78.3

0.96

52.2

* The regression for 2001 was not suitable to project the peak in gain/acre.

 

Table 4B shows what the gain/acre would have been each year if we had stocked at that rate. We predict that if we had stocked at 2.14 AUM/acre each year gain per acre would have ranged from a loss of 44.6 lbs/acre in 2002 to a gain of 148.9 lbs/acre in 1993 but would have averaged 78.3 lbs/acre. Because so little forage was produced in 2002 the grazing season was cut short and none of the pastures were actually stocked that heavily. The stocking rate on the extreme treatment averaged 1.18 AUM/acre for 2002 and, although gains were not good, only 11 out of the 189 animals in the trial had actually lost weight during the 55 days of grazing. Table 4C shows what the gain/acre would have been each year if the stocking rate were held constant at 0.96 AUM/acre, the average of the moderate treatment over this period.


Figure 4 shows the relationship between stocking rate and economic return. Cost for land, labor and management are not included because they vary greatly from one operation to another. If cattle prices were constant then return/acre would peak at a stocking rate somewhere below maximum gain/acre with the exact point depending on carrying costs (interest, death loss, salt and mineral, vet cost, transportation, labor and land). However, when cattle are worth more per hundred weight in the spring then they are in the fall it causes the point of maximum return/acre to occur at a lower stocking rate and when they are worth more in the fall it causes the maximum return to occur at a higher stocking rate. Again returns for stocking rates beyond the extreme stocking rate reference line are hypothetical and again all but 2001 had at least one observation from a stocking rate higher than the rate which would provide maximum return to land, labor and management for the year. Table 5A shows what the return/acre would have been if we had stocked each year at the stocking rate which would result in the maximum return for that year. These values correspond to the peaks of the curves in figure 4. Obviously we can’t know ahead of time what the optimum stocking rate for a particular year is going to be. If we were going to pick one constant stocking rate that would have provided the maximum return/acre over this last 12-year period it would have been 1.76 AUM/acre. This is the point labeled optimum in figure 4. Table 5B shows what the returns/acre would have been each year if we had stocked at this rate. Table 5C shows what returns/acre would have been each year if stocking rates were held constant at 0.96 AUM/acre, the average of the moderate treatment over this period. Although the average return per acre is higher under the optimum rate there were four years with negative returns while only one year had a negative return under the moderate stocking rate. (Costs for land, labor and management have not been subtracted). Comparing tables 4 and 5 it can be seen that in all but three years (1992, 1996 and 1999) the stocking rate with the greatest economic return was less than the rate with the greatest gain per acre.


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 12-year period.

Returns/acre to land, labor and management over the 12-year period if stocking rate were held constant at 0.96 AUMs/acre, 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

0.88

$4.10

38.8

1.76

-$1.60

59.4

0.96

$4.06

41.3

1992

3.12

$97.11

130.9

1.76

$77.87

118.4

0.96

$48.67

77.6

1993

2.39

$105.11

158.3

1.76

$97.64

130.1

0.96

$66.39

74.9

1994

0.67

$1.99

28.5

1.76

-$4.76

55.1

0.96

$1.52

38.2

1995

1.44

$2.05

59.5

1.76

$1.36

68.4

0.96

$0.49

42.5

1996

2.06

$31.763

56.9

1.76

$30.967

55.8

0.96

$21.60

38.9

1997

1.11

$13.35

73.5

1.76

$8.32

91.7

0.96

$13.06

65.6

1998

0.961

$2.11

63.1

1.76

$-3.46

88.7

0.96

$2.08

60.1

1999

3.24

$56.58

111.3

1.76

$43.46

87.0

0.96

$25.41

51.76

2000

2.19

$18.05

72.8

1.76

$17.22

65.7

0.96

$11.21

42.7

2001

 

*

 

1.76

$42.01

85.0

0.96

$25.59

50.8

2002

0.31

-$1.49

17.8

1.76

-$29.56

1.6

0.96

-$7.14

41.5

12 yr. avg.

1.67

$30.07

73.8

1.76

$23.30

75.6

0.96

$17.74

52.2

* The regression for 2001 was not suitable to project the peak in returns to land, labor and management.




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