Diet selection and foraging efficiency of Nguni goats in the Zululand Thornveld, Kwazulu-Natal

Abstract

Foraging efficiency and diet selection are central processes in plant-herbivore interactions. An understanding of these important parameters of foraging behaviour underpins sustainable vegetation management and profitable animal production. As a step towards better understanding the browse-browser interactions in a sub-humid subtropical savanna, a Nguni goat breed was used as a model browser in cafeteria-style experiments. The primary objective of the study was to determine the seasonal and plant species patterns of foraging efficiency (indexed by short-term intake rates) and diet selection (indexed by intake). The secondary aim was to investigate how potential explanatory variables such as plant morphology (spinescence, long vs. short shoots and broad vs. fine leaves), phenology (evergreen vs. deciduous species) and plant chemistry (nitrogen, neutral detergent fibre, acid detergent fibre, acid detergent lignin, condensed tannin, cellulose and hemicellulose) are related with the observed patterns.

Six browse species (Acacia natalitia, A. nilotica, Dichrostachys cinerea, Grewia occidentalis, Gymnosporia marangiiensis and Sciitia myrtind) were used in two experiments to estimate (1) short-term intake rates and (2) intake patterns at four times during the year (i.e. early, middle and late wet season and dry season). Intake rates varied significantly (P ~ 0.022) among plant species, with broad leaved species being consumed at higher rates than fine leaved ones. Intake rates were highest during the dry season (mean: 0.18 g s"1 ± SEM: 0.012 g s'1) and lowest in the late wet season (mean: 0.15 g s" ± SEM: 0.009 g s" ). Bite size was found to be the key variable influencing the intake rate of Nguni goats. No significant relationships were found between intake rate and either nitrogen, neutral detergent fibre, acid detergent fibre, acid detergent lignin, cellulose or hemicellulose concentrations. Condensed tannin concentration (% DM) was positively correlated with both intake rate (r = 0.65, P = 0.001, n = 24) and bite size (r = 0.53, P = 0.007, #i = 24). Diet selection was affected by the interactions between the seasons and browse species (P = 0.001). S. myrxina and G. occidentalis were consistently among the most preferred species through out all seasons, while the opposite was true for G. marangiiensis and A. nilotica. During the dry season, A. natalitia ranked third among the avoided species and inconsistently featured among the most preferred species later in wet seasons. The opposite was observed for D. cinerea. A positive relationship was also observed between intake (an index for diet selection) and condensed tannin (r = 0.27. P = 0.001, n = 144). Both foraging efficiency and diet selection were influenced more by the leaf and shoot morphology than spinescence. Plant chemistry did not clearly explain either foraging efficiency or diet selection patterns. These results have important implications for vegetation management and goat production in the Zululand Coastal Thornveld.