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 average above ground biomass production by grazing treatment on the silty range sites from 1992 to 1999. It can be seen that on average the light treatment has produced the most forage and the extreme treatment has produced the least. The heavy treatment produces less than all but the extreme treatment. The moderate grazing treatment produces more above ground biomass than the ungrazed treatment except at the beginning of the grazing season. Silty range sites have significant year X treatment interaction in forage production so treatment differences in specific years should also be discussed.

Beginning of season production decreased with increasing grazing intensity in 1992 and 1993 and also in 1995 except that the ungrazed treatment produced less than the light. 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 carbohydrates reserves that the plants were able to carry over to the next season. In 1997 the extreme treatment produced the least and the other treatments were not significantly different. In 1998 both heavy and extreme produced less than the other treatments. In 1999 the moderate produced less than the ungrazed and light, and the extreme and heavy produced less than the moderate.

Mid-season production decreased from light to moderate to heavy to extreme except that in 1994 the moderate produced more than light, and in 1999 the extreme produced more than the heavy but those differences were not significant. The ungrazed treatment went from being the most productive in 1992 to producing less than the light in 1993 to producing the least of any treatment in 1994. Its production was between moderate and heavy from 1995 to 1997. It was between moderate and light in 1998 and between moderate and extreme in 1999.

Table 6. Average above ground biomass production by grazing treatment on silty range sites from 1992 to 1999.
  Above Ground Biomass (lbs/acre)
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season
Ungrazed

Light

Moderate

Heavy

Extreme

LSD(0.05)

1,455 a1

1,488 a

1,286 b

927 c

741 d

118

2,590 b

2,959 a

2,700 b

2,234 c

1,829 d

198

2,909 b

3,309 a

3,099 ab

2,486 c

2,244 c

281

2,714 b

3,091 a

2,920 ab

2,353 c

2,216 c

305

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

Total biomass production for the season decreased with grazing intensity in 1992. There was no difference in total biomass production between treatments in 1993, but grass production was significantly less on the heavy and extreme treatments. This was the second wettest year during the study and the year production on silty was the greatest (see Table 2). In 1994 production on both the ungrazed and light treatments were suppressed. Soil water was adequate and there was an abundance of residual vegetation on these treatments from the previous years production. Sharrow and Wright (1977) working in tobasa grass 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. The moderate treatment produced the most biomass followed by heavy, light, extreme and ungrazed. In 1995 through 1997 the light treatment produced the most followed by moderate, ungrazed, heavy and extreme. There were no differences in total biomass for the season in 1998 and 1999.

End of season production was only significantly different between treatments by year in 1994 and 1997. In 1994 the moderate treatment produced the most and the ungrazed treatment produced the least. In 1997 the extreme treatment produced significantly less than all the other treatments.

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. 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 shows average above ground biomass production by grazing treatment on the overflow range sites from 1993 to 1999. It can be seen that generally the heavy or moderate treatment has produced the most biomass, followed by light with the extreme or ungrazed treatments producing the least. There was no significant year X treatment interaction in biomass production on overflow sites.

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. There seems to be a trend developing on the overflow sites for forb production to increase with increasing grazing intensity.

Table 7. Average above ground biomass production by grazing treatment on overflow range sites from 1993 to 1999.
  Above ground biomass (lbs/acre)
Treatment Beginning
of Season
Middle of
Season
Total
Yield
End of
Season
Ungrazed

Light

Moderate

Heavy

Extreme

LSD(0.05)

1,135 b1

1,161 b

1,319 a

1,353 a

899 c

145

3,347 b

4,184 a

4,129 a

4,227 a

2,472 c

459

3,396 b

4,346 a

4,613 a

4,565 a

2,962 b

456

2,676 c

3,895 b

4,371 ab

4,521 a

2,863 c

480

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

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. Second, percent disappearance was less than ideal disappearance for all but the heavy grazing treatment. Some adjustments will be made in the number of livestock and the location of salt and mineral blocks in some of the pastures in 2000 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 1999.
  Percent Disappearance1

Treatment Overflow Silty Average
For
Treatment
Ideal
Disappearance
for Pasture2
Ungrazed 43 1 -- --
Light 30 45 44 45
Moderate 42 45 43 55
Heavy 65 73 72 62
Extreme 77 83 82 85


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.

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.

Forage Quality

The forage samples were analyzed each year until 1999 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 have developed 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.