The effects of grazing, fire, nitrogen and water availability on nutritional quality of grass in semi-arid savanna, South Africa

Abstract

The impacts of fire and grazing management on grass nutritional quality in semi-arid savannas may depend on inherent variation in rainfall and soil nutrient status. We examined the effects of grazing, fire, nitrogen addition, and watering treatments on the nutritional value of grass in a field experiment in the Northern Cape, South Africa. Surprisingly, water addition had no effect on grass quality and biomass in this dry savanna, perhaps because of the above-average rainfall during this study. Nitrogen addition resulted in increased levels of crude protein and biomass, as well as crude protein and gross energy in the early and late wet seasons, respectively. Grazing alone generally had little effect on grass quality, although grass in grazed plots had higher levels of crude protein (CP) in the late wet season and phosphorus (P) in the dry season. Grass biomass was greater in plots protected from grazing. There was more CP and P mass per unit area in fenced (ungrazed) plots during the wet season. Fire interacted with addition of nitrogen and grazing to increase grass quality. Soil nitrogen availability appears to be the most important factor affecting nutrient quality of grasses in the wet season in this semi-arid ecosystem.