Drought and grazing patch dynamics under different grazing management

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. This study provides evidence that, in large paddocks in this environment, rotational grazing can reduce the deterioration and allow improvement of both shortgrass and midgrass patches.