ORIGINAL ARTICLE Chemical composition, nitrogen degradability and in vitro ruminal biological activity of tannins in vines harvested from four tropical sweet potato (Ipomoea batatas L.) varieties R. Ali 1 , V. Mlambo 2 , M. C. Mangwe 3 and B. J. Dlamini 3 1 Department of Food Production, Faculty of Food and Agriculture, The University of the West Indies St. Augustine, Trinidad & Tobago 2 Department of Animal Science, Faculty of Agriculture, Science and Technology, North-West University Mmabatho, Mafikeng, South Africa, and 3 Department of Animal Science, Faculty of Agriculture, University of Swaziland, Luyengo, Swaziland Summary This study investigated the potential of vines from four sweet potato varieties (Tia Nong 57, Tia Nong 66, Ligwalagwala and Kenya) as alternative feed resources for ruminant livestock. The chemical composition [neu- tral detergent fibre (NDF), acid detergent fibre (ADF), crude protein (CP) and acid detergent insoluble nitrogen (ADIN)], in vitro ruminal nitrogen (N) degradability and in vitro ruminal biological activity of tannins in the vines, harvested at 70 and 110 days after planting (DAP), were determined. Variety and harvesting stage did not (p > 0.05) influence CP and NDF content of the vines. Concentration of CP ranged from 104.9 to 212.2 g/kg DM, while NDF ranged from 439.4 to 529.2 g/kg DM across harvesting stages and varieties. Nitrogen degradabili- ty (ND) at 70 and 110 DAP was highest (p < 0.05) in Ligwalagwala (743.1 and 985.0 g/kg DM, respectively). Treatment of vines with tannin-binding polyethylene glycol (PEG) increased (p < 0.05) in vitro ruminal cumula- tive gas production parameters (a, b and c). The in vitro ruminal biological activity of tannins, as measured by increment in gas production parameters upon PEG inclusion, had a maximum value of 18.2%, suggesting low to moderate antinutritional tannin activity. Ligwalagwala vines, with highly degradable N, would be the best pro- tein supplement to use during the dry season when ruminant animals consume low N basal diets and mainte- nance is an acceptable production objective. Tia Nong 66 and Kenya varieties, with less degradable N, may be more suitable for use as supplements for high-producing animals such as dairy goats. Keywords foliage, harvesting stage, nutritive value, antinutritional effects, in vitro ruminal fermentation Correspondence V. Mlambo, Department of Animal Science, Faculty of Agriculture, Science and Technology, North-West University, Mmabatho, Mafikeng 2735, South Africa. Tel: +27-18-3892741; Fax: +27-18-3892748; E-mails: Victor.Mlambo@nwu.ac.za; victormlambo@yahoo.co.uk Received: 29 December 2014; accepted: 9 March 2015 Introduction Sweet potato (Ipomoea batatas L.) is one of the five most important food crops, particularly under subsis- tence farming in many developing countries (Phuc et al., 2001). It is known for its high productivity and low input requirements (Claessens et al., 2009). The crop is mainly grown for tubers, but a large volume of sweet potato vines (stems and leaves) are left after harvesting. The vines are a potential feedstuff for live- stock (Nyaata et al., 2000); however, their optimum utilization as feed is constrained by a lack of empirical data on their nutritive value. In particular, the effect of genotype and stage of maturity of sweet potato plants on nutritive value needs to be investigated. The vines can be used as an alternative feed resource for ruminants to reduce competition between the animals and humans for grains (maize and soya bean). The value of vines as feed supplements is enhanced by high palatability, moderate levels of crude protein (11–17%), high levels of digestibility (>62%) and high moisture content (Ffoulkes et al., 1997). The major limiting nutrient for ruminant animals during the dry season is protein; therefore, the ruminal fermentation characteristics of protein in sweet potato vines need to be evaluated. The suitability of the vines as a protein supplement will depend on the extent to which N is degraded in the rumen. It is important to note that ruminal degradability of N in sweet potato vines may be affected by tannins resulting in positive or negative nutritional outcomes depending on diet and level of productivity of animals. Indeed, Phale and Madibela (2006) reported that sweet potato vines have up to 67.7 g/kg DM soluble condensed tannins. Very little Journal of Animal Physiology and Animal Nutrition © 2015 Blackwell Verlag GmbH 1 DOI: 10.1111/jpn.12329