Funding:
E. Paul and Helen Buck Waggoner Foundation, Inc.
USDA Agricultural Systems Grants 9404256 and 98-35108-6491
Texas AgriLife Research.

Project Duration September 1996 – August 2000

Land deterioration does not occur uniformly over time or over a landscape. The differential use of preferred areas in the landscape results in uneven distribution of animal impact, and periods of below average precipitation compound the effects of herbivory, providing periods of accelerated deterioration. This study investigates whether rotational grazing during a drought cycle allows reduction of deterioration caused by patch-selective grazing in large (1800-2100 ha) paddocks by providing adequate rest between grazing events. From 1995 through 2000, herbaceous and bare ground changes were measured on adjacent heavily grazed and lightly grazed patches in rotationally and continuously grazed paddocks.

The weather interacted with grazing treatment (p < 0.0001), species (p < 0.0001) and the combined effects of the other factors (p < 0.0014), indicating the dominant effect of weather, particularly precipitation, on changes in herbaceous basal area. When summer growing conditions were favorable, the rotational grazing treatment resulted in greater increases of perennial herbaceous basal areas (p < 0.05) and lower proportions of bare ground (p<0.10) than the continuously grazed treatment. Although rotational grazing did not prevent deterioration in basal area and bare ground with the series of four drought years, it did decrease the rate of deterioration. The changes in basal area were primarily due to changes in summer growing perennial C4 midgrasses and C4 shortgrasses. Grazing treatment did not influence species aerial biomass composition (p > 0.1). When monitoring to effect sustainable use, the commonly used parameter of species composition appears to be a much less sensitive indicator of change than bare ground and basal area.

To sustain the range resource, it is of paramount importance that management practices minimize bare ground, increase herbaceous perennial basal area, and decrease the area of patches dominated by low successional species. The process of patch selective grazing induces a micro-pattern of heavily grazed areas and lightly or ungrazed areas. The heavily grazed areas are overgrazed even when the stocking rate for the grazing unit as a whole is within the carrying capacity. By providing periodic rests from defoliation, rotational grazing management has the potential to minimize the effects of patch overgrazing. Much research indicates that patterns of selection are not changed with different management systems. However, research comparing continuous with rotational grazing has only been conducted on small areas of 5 to 25 ha, which totally ignores the documented patterns of selection and resulting effects that occur in large paddocks. The relatively small research paddocks which are grazed continuously to compare with rotational grazing do not mimic the continuous grazing of large paddocks as intended because they preclude the manifestation of a critical element of continuous grazing, namely uneven utilization over the landscape. The conclusions of such research have de facto been extrapolated to all pastoral situations, regardless of paddock size.

This study indicates that in large paddocks, rotational grazing can reduce degradation caused by patch overgrazing. To prevent the deterioration of heavily grazed patches, adequate lengths of time between successive defoliations must be provided. Planned rotational grazing is the tool that provides managers with the opportunity to address the root cause of patch overgrazing and deterioration. It is, therefore, a key tool in managing for sustainable use of rangeland.

References

Teague, W.R. S.L. Dowhower and J.A. Waggoner. 2004. Drought and grazing patch dynamics under different grazing management. Journal of Arid Environments 58:97-117.