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.

Uittreksel

Die Jones‐Sandlandtipe model, wat in algemene gebruik in suidelike Afrika is. mag gekritiseer word omdat dit eerstens die langtermyn effek van beweidingsintensiteit op die smaaklikheid en produktiwiteit van die weiding of veld, en tweedens moontlike onderbrekings in die verwantskap tussen dierprestasie en veebelading ignoreer. ‘n Wiskundige model om hierdie sake in suurgrasvekl te ondersoek, is ontwerp. Van die model en onafhanklike weidingsproewe is dit afgelei dat daar ‘n kritiese veebelading is waarbo die beweidingsintensiteit ‘n veldagteruitgaan en ‘n verandering in die diereprestasie‐veebelading verwantskap veroorsaak. In veld van uitstekende toestand hierdie kritiese veebelading ooreenstem met die van maksimale diereproduksie per ha. Nogtans op slegte veld is die kritiese veebelading minder as die veebelading wat maksimale diereproduksie per ha oplewer. Vernederde suurgrasveld wat vir diereproduksie doeleindes gebruik is, het dus herstelvermoe — beweidings‐intensteit gaan waarskynlik die drumpelwaarde waaronder veldverbetering moontlik is, dikwels oortref. Wat betref die onderbrekings, bestaan hulle nie. Nogtans is daar ‘n noue omvangs van veebelading waarin die tempo van verandering in dierprestasie met betrekking tot die tempo van verandering in veebelading groot is. Om die Jones‐Sandland model aan te neem sal dus hoogs veranderlike resultate tussen replikasies en seisoene oplewer.