Evaluation of potential and effective rumen digestion of mistletoe species and woody species browsed by goats in a semi-arid savanna, southwest Zimbabwe

The potential and effective rumen digestion of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis and Viscum verrucosum) and four acacia species (Acacia gerrardii, Acacia karroo, Acacia nilotica and Acacia robusta) browsed by goats in the semi-arid savanna in Zimbabwe was determined in vitro with or without including polyethylene glycol (PEG). The in vitro gas production (IVGP) (74.6 vs. 63.1 ml), gas production rate (0.05/h vs. 0.035/h) and effective degradability (46.7 vs. 35.3 ml) were higher in mistletoe than acacia (P<0.01). For mistletoe species, P. kalachariensis had the highest IVGP, potential gas production (b), gas production rate (c) and effective degradability (ED) than E. ngamicum and V. verrucosum (P<0.01). In acacia species, IVGP and potential gas production (b) were higher in A. karroo whilst A. gerrardii had the greatest gas production rate (c) than the other acacia species. In all browse species, addition of PEG, which minimised the inhibitory effects of tannin on microbial fermentation, resulted in an increase in gas production parameters except in P. kalachariensis (P<0.01). The effect of PEG on fermentation and degradability was greater in acacia foliage than in mistletoe foliage (IVGP, 19 vs. 41%; potential gas production, 11 vs. 16%; effective degradability, 13 vs. 42%), which suggested that the tannin in acacia was more biologically active than that in mistletoe. In both mistletoe and acacia species, there was an interaction between species and PEG for IVGP and effective degradability. This result suggested that the effects of PEG on fermentation parameters were species-specific as species responded differently to the addition of PEG. The increase in fermentation parameters due to the addition of PEG varied widely amongst the acacia species (range: 10–114%) and mistletoe species (range: 7–49%). Thus, the inclusion of PEG to neutralise or reduce the effects of tannins should consider the species were its use would be more beneficial, such as in A. gerrardii, A. robusta, and E. ngamicum in this study. In conclusion, mistletoe species show a high nutritive value with large potential for feeding goats in the semi-arid savanna due to the higher values of IVGP, gas production rate, potential gas production, and effective degradation than those for acacia species. In addition, our results support the use of PEG to neutralise tannins in tannin-rich forages.