Supplemental nutrients increase the consumption of chemically defended shrubs by free-ranging herbivores

Abstract

Large herbivores are purported to continue consuming toxin-containing forages as long as their capacity to neutralize, detoxify and excrete dietary toxins is not exceeded. This capacity depends on the availability of liver enzymes, energy and amino acid precursors. While this may explain increased intake of toxin-rich forages by herbivores supplemented with nutrients, a different effect may emerge in rangelands dominated by forages that are rich in condensed tannins which are not as degradable and readily absorbable as toxins. In a field experiment, we investigated the effects of supplementing animals with a high-energy source (yellow maize grain) and a high-protein source (soybean meal) on browse intake, foraging behaviour and diet composition of goats in a semi-arid savanna. In line with our prediction, nutrient supplementation increased the percentage time spent by goats on browsing and subsequently enhanced browse intake. Supplemented goats consumed more condensed tannins than goats that were not supplemented. Goats from supplemented groups tended to compose different diets from goats that received no supplement. Supplementation may have modified animals’ requirements for nutrients in ways that influenced feeding behaviour. We contend that supplements likely replaced the nutrients that are routinely bound and rendered indigestible by condensed tannins and thus increased the intake of tannin-rich forages through delaying a negative post-ingestive feedback (aversion) from dietary tannins. In conclusion, we showed that nutrient supplementation can be used to increase defoliation of tannin-rich shrubs by mixed feeders which suggests a potential for browsers and mixed-feeders to serve as biological bush control agents. Furthermore, these results have serious implication for herbivores and the environment.