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Ntuthuko Mkhize studies how condensed tannins in plant leaves influence the foraging and ultimately growth of herbivores, using an experiment with free-ranging goats in South Africa.
Effects of Plant Secondary Metabolites on Large Herbivores: Implications for Bush Control in African Savannas.
Using a goat (Capra hircus) as a model browser, I study how condensed tannins (as examples for PSMs) influence browse resource utilization by large herbivores. I designed field experiments that would allow direct observations of foraging behavior in Roodeplaat Farm, South Africa. This study involves orally dosing animals daily with condensed tannins and polyethylene glycol before observing their diet composition over an extended period of time. Vegetation is regularly surveyed to estimate changes in forage quantity and quality during the study and also to be able to explain diets selection in terms of chemical and physical plant traits over time.
In another field experiment, I test the hypothesis that “herbivores increase the consumption of plants containing PSMs as long as their capacity to neutralize and excrete these toxins is not exceeded”. Fieldwork involves supplementing animals with macro nutrients as a way to enhance their ability to detoxify and probably increase their use of chemically defended woody plants. Although this has been successfully proven under simple, single-species systems, its success has not yet been tested in the complex heterogeneous savanna environments.
Herbivores generally make complex foraging decisions with a goal to maximize their fitness and reproduction. Long-term effects of PSMs on free-ranging herbivore productivity are not well understood. This study addresses a specific question: “How do condensed tannins influence growth performance of free-ranging goats? Body weights, nitrogen excretion and nutritionally related blood metabolites are periodically measured from free-ranging goats orally dosed with condensed tannins and polyethylene glycol.
A clearer understanding of how chemical characteristics of woody plants interact with herbivores presents an opportunity (1) for improving herbivore productivity and (2) to guide future modeling of browse-browser interactions in rangelands dominated by chemically defended encroaching woody plants.