Determining spatial and temporal variability in quantity and quality of vegetation for estimating the predictable sustainable stocking rate in the semi-arid savanna

Abstract

This study assessed the importance of spatial and temporal variation in plant quality and quantity for determining sustainable stocking rates in game, commercial and communal ranches in semi-arid savanna of the Northern Cape Province, South Africa, in wet and dry seasons over a two-year period. We focussed on variation in plant biomass, phosphorus (P), crude protein (CP) concentrations and dry matter digestibility as parameters most likely to affect sustainable stocking rates. Habitat type had greater effects on plant quality, plant biomass and species composition than management type. The commercially-managed area had the highest tree density in the rocky habitat and lower plant quality than other management types. All of these features indicate that land degradation is occurring on commercial ranches in spite of rotational grazing and lower stocking density than on communal ranches. We recommend that commercial ranchers should introduce a greater variety of stock and/or game to reduce selective grazing of certain plant species. Quality measures (CP and P) gave more conservative predictions of stocking density than biomass. In this region of the Northern Cape, seasonally-inundated pan habitats are particularly valuable in spite of low-standing crop because they have the highest year-round quality. Contrastingly, ranchers should only lightly stock open savanna habitats, in spite of high standing biomass, because they have low vegetation quality and may be particularly susceptible to degradation and invasion by poisonous and unpalatable plants.