Heterogeneity in tall fescue pastures created and sustained by cattle grazing.

Abstract

In continuous grazing systems the amount of herbage and its quality is a matter of primary concern. However, at moderate stocking, cattle grazing usually leads to the generation of patches differing in forage quality and quantity even in virtually monospecific pastures. This patchiness influences subsequent vegetation and animal responses. We analyzed the heterogeneity created and sustained by cattle grazing in a tall fescue (Festuca arundinacea Schreb.) pasture at stocking densities ranging from 1.8 to 4.1 animals ha-1 over 2 years. Cattle grazing created and maintained a mosaic of areas with different degrees of utilization. Heavily utilized patches had less biomass per unit surface, but their live biomass was more dense and had a higher nitrogen concentration. Patch boundaries fluctuated throughout the year at all stocking densities. Patch locations were more stable at the lower stocking densities, where cattle repeatedly returned to heavily utilized patches even though they represented less than 30% of the total surface. This reinforces the idea that, at low and moderate stocking densities, cattle can obtain a nutritional benefit by patch grazing. The percentage of heavily utilized patches reached a maximum value at an instantaneous grazing pressure of approximately 0.0016 animal units kg forage-l. When this threshold is passed, animal selection between patches could be conditioned by the presence of feces or thistles, and pasture condition affected by overgrazing of the heavily utilized patches.