OVERGRAZING - PRESENT OR ABSENT

This paper discusses the criteria needed for quantitative evidence of overgrazing and outlines some of the main pasture and external factors that promote overgrazing by herbivores. Overgrazing is defined as occurring where there is a concomitant vegetation change and loss of animal productivity arising from the grazing of land by herbivores. Confirmation of the loss of productivity requires the measurement of departures from the linear relationship between animal productivity and stocking rate for any given animal-pasture system. In the ex-ante situation of an experiment, overgrazing will be observed as a loss of linearity with time. In the ex-poste situation of a comparison between 2 paddocks of the same range type, but different grazing history, overgrazing will be observed as a difference in the optimum stocking rate. The outcome of a species change in terms of productivity is shown to be complex because of the interaction of the quality and quantity influences in both pasture and product. Influences that promote lower stocking rates include low cost-price margins and a negative relationship between product quality and grazing intensity. Conversely, higher stocking rates are promoted by the use of mineral supplements and products such as wool that have a positive relationship between product quality and stocking rate.