干旱区生态环境知识资源中心

The influence of stocking rate on the ecological and economic sustainability of grazing systems is discussed. The maximum, long-term stocking rate that does not cause pasture degradation is called the maximum sustainable stocking rate (SSR) to avoid confusion with two related but poorly defined terms, carrying capacity and grazing capacity. SSR is based on critical levels of utilisation which should be qualified by a probability rating for the reliability of local climate. Levels of critical utilisation differ across pasture types, being lowest in ecologically sensitive arid systems and highest in sown temperate pastures. Production per animal against stocking rate was expressed by a 3-phase linear model for a wide range of grazing systems. Each phase had both biological and managerial implications: Phase 1 was the potential production per animal; Phase 2 exhibited a linear decline in production per animal as stocking rate increased; and Phase 3 displayed the influence on production per animal of changes in botanical composition at high stocking rates. The junction of Phase 2 and 3 indicated SSR. Also, the slope of the Phase 3 indicated the impact of pasture degradation on animal production. The slope increased sharply when degradation was accompanied by loss of basal cover and increase in weeds, but the change in slope was much less when stoloniferous species replaced tufted species.

The relative magnitude of stocking rates for highest profit and SSR reflected the likelihood of profit maximisation leading to pasture degradation. Evidence suggested that for beef production, where carcass quality and price are inversely related to stocking rate, the most profitable stocking rates are less than SSR. However, when fibre quality and price of product are directly related to stocking rate, the most profitable stocking rates for fibre production are likely to approach SSR. Both production systems offer flexible management, since near maximum profits occur over a range of stocking rates. Reports from several farmers support the view that low stocking rates enhance ecological and economic sustainability of grazing systems in variable climates.

The knowledge, skill and attitudes of a manager are key elements in sustainable conditions. risks associated with high stocking rates are more difficult to avoid and land degradation is more likely, particularly when variable costs are perceived to be low. Ecological sustainability should benefit from destocking as a drought management strategy, rather than supplementary feeding. Public demands are increasing for grasslands to be a source of fresh water and a control measure for emission of greenhouse eases. Both processes are influenced by stocking rate.