Effects of season and breed on browse species intake rates and diet selection by goats in the False Thornveld of the Eastern Cape, South Africa

Abstract

Although goats are common domestic browsers in African savannas, factors that affect their diet selection and food intake rate are poorly understood. This study used a cafeteria-style experimental procedure to investigate the effects of season and breed on browse species intake rates and browse species preference by Boer and Nguni goats in the False Thornveld of the Eastern Cape. Six browse species that commonly occur in the study area were used: Grewia occidentalis L. (Tiliaceae), Scutia myrtina (Burm. f.) Kurz (Rhamnaceae), Diospyros lycioides Desf. subsp. lycioides (Ebenaceae), Rhus longispina Eckl. and Zeyh. (Anacardiaceae), Ehretia rigida (Thunb.) Druce (Boraginaceae) and Acacia karroo Hayne (Fabaceae=Leguminosae). Cut branches were offered to goats during winter and summer and instantaneous intake rates and preferences were measured. There were no significant differences between Boer goats and Nguni goats in dry matter intake rates per unit metabolic body weight and in preferences of the six browse species, suggesting a potential for competition should the resources become limiting. However, the intake rates of Nguni goats increased by 50% from winter to summer, while intake rates of Boer goats increased by 87%. The mean intake rate was 69% higher in summer than in winter. Intake rates varied among all browse species, except A. karroo and R. longispina. Intake rates of D. lycioides, G. occidentalis and S. myrtina were higher than those of the other three species. Intake rates tended to increase more among deciduous species than evergreen species in summer, but the effect was not consistent. Likewise, intake rate within seasons was not consistently related to leaf phenology; intake rate of deciduous species was neither lower nor higher than that of evergreen species at any time. However, the increased intake rate in summer was consistent among those deciduous and evergreen species that produce all their new leaves on new long shoots, as opposed to species that produce their new leaves on old short shoots. The consequence of the two shoot morphologies for the browsing animal is that browseable material in the former group consists of shoots and leaves, but in the latter group consists mostly of leaves only. G. occidentalis and S. myrtina were the two most preferred species. Deciduous species were not consistently more preferred than evergreen. In accordance with optimal foraging theory, nutrient intake rates consistently explained preference in winter and summer. Species that offered the highest rates of nutrient intake were most preferred.