Browse production and offtake by free-ranging goats in an arid zone, Kenya

Degradation of woody plants in arid zone ecosystems is often blamed on browsing by free-ranging goats in and around settlements. However, lack of empirical evidence makes such a notion misleading to the management of tree regeneration. Five techniques were compared for the monitoring and estimation of browse offtake from young Acacia tortilis by free-ranging goats and the techniques were used to estimate goat relative stocking density in Kenya, an arid zone, from 1986-1990. The findings showed that A. tortilis uses compensatory browse production expressed through twig growth and herbivory tolerance to modify goat browsing pressure. Goat browsing did not result in different tree growth rates between the browsed and the unbrowsed treatments. Browsed trees produced more twigs than unbrowsed trees. The browsed twigs produced greater net cumulative twig growth than the unbrowsed, suggesting that goat herbivory stimulates browse production. Twig availability declined as trees grew out of reach of goats (i.e. > 150 cm). The five techniques estimated variable relative stocking densities at higher browse offtake, but provided comparable values when the browse offtake declined. The findings also showed that free-ranging goats adjusted browsing pressure in response to the decline in available browse and browsing efficiency, as opposed to intensifying browsing pressure as commonly believed. These results highlight the futility of establishing fixed stocking densities in arid environments where variability of browse production is great. Rather, resource managers may be obliged to calculate stocking densities yearly or seasonally.