Crude protein changes on grassland along a degradation gradient in a semi-arid climate

The aim with this investigation was to quantify the impact of different veld condition classes, viz. poor, moderate and good on soil-water utilization (SWU: crude protein produced per unit of evapotranspiration), during four growing seasons (2000/01 – 2003/2004). Evapotranspiration was determined by quantifying the soil-water balance equation with the aid of runoff plots and soil-water content measurements. Crude protein (CP) content was calculated from measured nitrogen content (Kjeldahl method) of the leaves, stems and seed. Although the monthly CP concentration (g/kg) of grassland in poor condition was generally higher than that of grassland in good condition, CP production decreased significantly with veld degradation when expressed as total quantity (kg/ha) of dry matter produced. Grassland in good, moderate and poor condition produced on average over the season 141, 97 and 32 kg CP/ha, respectively. Both monthly and seasonal SWU declined significantly with grassland degradation. Grassland in good condition averaged a SWU of 0.32 kg CP/ha/mm during the four growing seasons, compared to the only 0.09 kg CP/ha/mm produced from veld in poor condition. The study shows that it is important to keep grassland in optimal condition to utilize limited soil water for sustainable plant and therefore animal production. Keywords: Crude protein; evapotranspiration; soil-water content; soil-water utilization; veld condition South African Journal of Animal Science Vol. 36 (5) 2006: pp.34-37