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Wageningen University (2015)

Unlocking resources in savannas : how goats and other mixed feeders overcome the negative effects of tannins

Mkhize, N.R.

Titre : Unlocking resources in savannas : how goats and other mixed feeders overcome the negative effects of tannins

Auteur : Mkhize, N.R.

Université de soutenance : Wageningen University

Grade : Doctor 2015

Résumé
This thesis contributes insights on how condensed tannins might mediate the interactions between woody plants and large herbivores in the African savannas. Current understanding in this regard is still based on data from short-term laboratory experiments, mostly with confined animals and a few correlative field studies that only explore relationships between tannin concentrations of plants with their intake. Although these experiments are a necessary first step in isolating and characterising the effects of condensed tannins, they oversimplify the complex interactions that occur between wild herbivores or livestock and plants. The challenge for research is to translate the roles of tannins in plant-herbivore interactions from controlled experiments to field conditions. The aims of this research were : to (1) investigate how condensed tannins influence foraging behaviour and growth performance of free-ranging ruminant herbivores, and (2) determine the effects of supplements on use of woody plants and intake rates of condensed tannins by free-ranging herbivores in a semi-arid savanna.
Field experiments were conducted and data collected on free-ranging goats as models for all mixed feeders that share similar characteristics with goats. Condensed tannin exposure levels to goats were experimentally increased in the field by orally dosing 15 goats with 20g condensed tannin powder extracted from a bark of tannin-rich species. To reduce tannin exposure, 15 goats were dosed with 20 g of PEG in an attempt to neutralize tannin effects, and another group of 15 goats was dosed only with water and served as a control group. Feeding behaviour of goats supplemented with a protein-rich source, an energy-rich source were compared with that of goats that were not supplemented.
The results indicated that mixed feeders exposed to high levels of condensed tannins spend more time grazing and less time browsing compared to animals with low tannin exposure. However, the findings did not support expectation for tannins to reduce overall foraging time. Therefore, it was concluded that condensed tannins do not necessarily suppress foraging, but only influence the amount of time animals spend foraging on either herbaceous or woody forage. These findings also supported hypothesis that herbivores forage in ways that minimize their intake rate of condensed tannins. Animals altered their foraging behaviour depending on the treatment groups they were allocated to, and compiled diets that indicated tannin minimization as a goal. Moreover, there was support for the notion that condensed tannins are digestibility reducers. It was clear that free-ranging animals are able to employ their behavioural adaptations to chemical defences in ways that mitigate the negative physiological effects on their presumed ultimate fitness. This thesis presents possible effects of nutrient-tannin/toxin interactions on herbivores in African savannas. In the supplementation experiment, proteins and energy equally increased browse consumption by herbivores, with a concomitant increase in tannin intake rates.
These results were explained in light of the ongoing bush encroachment in the African savannas. The expected increase in the availability of browse will probably impose a selection pressure for herbivores that can better utilise the encroaching woody plants known to be endowed with tannins and other carbon-based secondary metabolites. These results are used to generalise about the herbivore health, herbivore nutritional, and environmental benefits that are possible from managing our rangelands and herbivores in ways that increase utilization of chemically defended plants. For example, increased consumption of tannin-rich forage will not only improve nutrition, but it will also reduce internal parasite burden, and reduce bloating by ruminant herbivores while simultaneously reducing the methane emissions that lead to global warming.

Mots clés : goats - tannins - animal nutrition - animal feeding - savannas - browsing - grazing time - grazing - animal physiology - plant composition - africa

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Page publiée le 11 décembre 2016, mise à jour le 14 janvier 2018