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Grass yields in this semi?arid savanna declined as the size of Acacia karroo increased. Browse yields, however, did not increase as trees grew out beyond 1,8 m in height and it follows that if bush thinning is required, then the taller trees should be selectively removed from these savannas. Simulated browsing of A. karroo trees was found to stimulate browse production, provided it was not too intense. As a consequence, the competitive ability of the trees increased, and grass yields were adversely affected. Conversely, simulated grazing reduced the competitiveness of the grass and, thereby, resulted in an increase in browse production. It is argued that residual soil moisture levels remain relatively high when grass growth is poor, so that water penetrates to greater depths after rain than when grass growth is vigorous and this favours the deep rooted trees. Livestock programmes in Thornveld should take account of the interacting effects of grazing and browsing on grass and browse yield in determining the stocking rates of grazers and browsers.