Combined conservation of jack bean and velvet bean with sorghum: evaluation of lab-scale silages and in vitro assessment of their nutritive value

Abstract

SUMMARYThe potential of ensiled sorghum–soybean as a ruminant feed has already been demonstrated; however, alternatives for soybean should be considered because other legumes such as jack bean (JB; Canavalia ensiformis (L.) DC.) and velvet bean (VB; Mucuna pruriens (L.) DC) might produce better yields under tropical conditions. First, the possibility for a qualitative conservation of these legumes in combination with sorghum was studied using lab-scale silages. Furthermore, the potential of additional molasses as a source of water-soluble carbohydrates (WSC) and a microbial inoculant (BIOPRANAL) to improve silage quality were assessed. As sorghum or legume tannins may influence the silage nutritive value, their importance was assessed through the addition of polyethylene glycol (PEG), which reduces tannin activity. Therefore, 25 treatments per legume–sorghum combination were created according to a central composite design. An acceptable silage quality was reached when at least half of the fresh biomass consisted of sorghum in combination with at least 15 g of additional WSC/kg fresh material. The nutritive value of mixed silages as well as pure sorghum silage in combination with dry jack beans was determined through in vitro digestibility in the rumen and small intestine. Polyethylene glycol increased the in vitro production of short-chain fatty acids, ammonia (NH3) concentrations and effective rumen dry matter and crude protein degradability for both mixed silages, indicating that tannins reduced rumen degradability. The latter action could be positive as the amount of digestible bypass protein increased when no PEG was added in the silo. Propionate and valerate proportions were increased through PEG addition to sorghum–JB and sorghum–VB silages, respectively, suggesting that the nature of the tannins differ between these legumes.