Stability, resilience and animal production in continuously grazed sour grassveld.

Abstract

The Jones-Sandland model, popularly used in southern Africa, may be criticised because it ignores firstly the long-term effects of grazing intensity on the acceptability and productivity of pasture or veld, and secondly possible discontinuities in the animal performance - stocking rate relationship. A mathematical model is developed to investigate these issues in sour grassveld. From the model and independent grazing trials, it is concluded that there is a critical stocking rate above which grazing intensity induces veld deterioration and change in the animal performance - stocking rate relationship. In veld in excellent condition this critical stocking rate coincides with that maximising animal production per ha. However, on poor veld the critical stocking rate is less than the rate yielding maximal animal production per ha. Degraded sour grassveld which is being used for animal production is therefore resilient - grazing intensity is likely often to exceed the threshold below which veld recovery is possible. Regarding discontinuities, these do not occur. However, there is a small range of stocking rate over which the rate of change in animal performance is extreme in relation to the rate of change in stocking rate. Adopting the Jones-Sandland model would thus yield highly variable results between replicates and seasons.

Keywords: animal performance; animal production; grassveld; grazing; grazing intensity; mathematical model; model; performance; productivity; southern africa; stocking rate; yield