Nutritive value and digestibility of mistletoes and woody species browsed by goats in a semi-arid savanna, southwest Zimbabwe

Abstract

The study examined the nutritive value 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. All were analysed for crude protein (CP), acid detergent fibre (ADF), neutral detergent fibre (NDF), acid detergent lignin (ADL), total phenolics (TP), condensed tannin (CT) and in vitro dry matter digestibility (IVDMD). In vitro gas production (IVGP) was measured with and without the addition of polyethylene glycol (PEG). In vitro metabolizable energy (IVME) was estimated from CP and IVGP. Acacia foliage contained more CP (120.9 vs. 109.5g/kg DM) and ADF (219.1 vs.161.1g/kg DM) content than mistletoe foliage. However, the content of ADL, TP, CT and IVDMD did not differ (P>0.05) between the two plant groups. In vitro dry matter digestibility differed significantly (P<0.01) among species within both mistletoe and acacia foliage, and the differences reflected variation in their chemical composition. The IVDMD for E. ngamicum, P. kalachariensis and V. verrucosum was 563.3, 537.3, and 339.2g/kg DM, respectively. In acacia species, the respective IVDMD for A. gerrardii, A. karroo, A. nilotica, and A. robusta was 604.2, 317.7, 509.1, and 479.6g/kg DM. Mistletoe foliage had higher in vitro ruminal fermentation parameters (i.e., IVGP and IVME) than acacia foliage. The IVGP without and with the addition of PEG was 62.2 and 68.5ml/300mg DM and 32.9 and 54.1ml/300mg DM in mistletoe and acacia foliage, respectively. The respective IVME in mistletoe foliage was 16.9 and 17.8MJ/kg DM and in acacia foliage, 13.6 and 16.4MJ/kg DM. The inclusion of PEG in the fermentation experiment increased IVGP and IVME in all acacia species and two mistletoe species (E. ngamicum and V. verrucosum). However, in P. kalachariensis inclusion of PEG caused a significant reduction in IVGP (−15.6%, P<0.001) and IVME (−9%, P<0.001). Overall, the inclusion of PEG caused a greater change in IVGP (64.4 vs. 10.1%), and IVME (20.6 vs. 5.3%) in acacia foliage than in mistletoe foliage. In vitro dry matter digestibility and fermentation parameters (i.e., IVGP and IVME) had inverse relationships with ADF, ADL, CT, and TP. In conclusion, the mistletoe species were of good fermentation quality than acacia species due to low ADF content and tannin activity. In this regard, mistletoe species show potential as alternative browse for goats in the semi-arid savanna.