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Long-Term Grazing Intensity Research in the Missouri Coteau of North Dakota

 

Effect of Grazing Intensity on Forage Production and Utilization

 


 

During 1992, the amount of forage produced under the various grazing treatments was significantly different on silty range sites. During 1993, the same was true on overflow range sites. Prior to these years, the amount of forage produced under the different grazing treatments had not differed significantly. Tables 6 and 7 list the average forage production by treatment. For both types of range sites, the extreme grazing treatment produced the least forage. However, the ungrazed treatment is not the most productive. The light treatment is the most productive on silty range sites. The light, moderate, and heavy treatments show little difference on overflow range sites, but the moderate grazing treatment tends to be the most productive. No year-X-treatment interaction between treatments on overflow range sites has been found. That means the effect of grazing intensity on forage production is consistent across all of the study years. However, there is year-X-treatment interaction on silty range sites, which indicates that the weather for the year, or from the previous year, may affect which grazing treatment produces the most forage. At the beginning of the grazing season, the most productive site varied between the ungrazed, light, and moderate treatments, with the extreme or heavy treatments always proving to be least productive. At the middle of the season and in peak yield, the light treatment produced the most forage or was not significantly different from the most productive treatment and the extreme treatment produced the least forage or was not significantly different from the least productive treatment. At the end of the season, the light or moderate treatment was most productive and the extreme or heavy treatment was least productive, although the ungrazed treatment produced the least forage in 1994, and the mean for the ungrazed treatment was greater than for the light treatment in 2002.

 

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 the plants were able to carry over to the next season. Part of the variability in production on the ungrazed treatment may be the result of litter buildup that can prevent rainfall and sunlight from reaching the ground. In 1992 and 1993, the ungrazed treatment produced the most forage on silty range sites and production decreased as grazing intensity increased. Annual rainfall in 1993 was the second highest of any year during the study and the greatest forage production on silty range sites occurred during that year (see Table 2). The buildup of litter in that year may have caused the ungrazed treatment to be the least productive treatment in 1994.

 

Table 6. Average above-ground biomass production by grazing treatment on silty range sites from 1992 to 2008.

 

Above-ground biomass (lbs/acre)

Treatment

Beginning

of season

Middle of

season

Peak

yield

End of

season

Ungrazed

  1,265 a1

2,400 b

2,683 c

 2,537 c

Light

 1,318 a

 2,748 a

3,116 a

 3,008 a

Moderate

 1,183 b

  2,519 b

2,892 b

 2,785 b

Heavy

  890 c

 2,136 c

2,391 d

 2,299 d

Extreme

  726 d

  1,815 d

2,152 e

2,109 d

LSD (0.05)

65

145

184

200

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

 

 

Table 7. Average above-ground biomass production by grazing treatment on overflow range sites from 1993 to 2008.

 

Above-ground biomass (lbs/acre)

Treatment

Beginning

of season

Middle of

season

Peak

yield

End of

season

Ungrazed

    990 c1

3,214 b

3,305 b

2,864 b

Light

1,154 b

3,833 a

4,104 a

3,841 a

Moderate

1,248 a

3,708 a

4,141 a

4,004 a

Heavy

 1,244 ab

3,604 a

3,934 a

3,896 a

Extreme

   857 d

2,240 c

2,616 c

2,524 c

LSD (0.05)

84

262

273

284

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

 


Soil Water and Forage Production

Soil water has been sampled bi-monthly throughout the growing season on each of the vegetation monitoring sites, and differences in available water have developed 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.


Forage Quality

The nutritional quality of the forage was sampled at the middle of the grazing season each year for the first 10 years of the study. 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 remained ungrazed. These differences in nutritional quality have occurred gradually over the course of the study



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