A review of anti-nutritive factors limiting potential use of Acacia angustissima as a ruminant feed

Acacia angustissima is a tropical legume which originated from Central America and has potential as a protein supplement to improve ruminant nutrition if the adverse effects of anti-nutritive factors can be diminished. It has a high nitrogen content and possesses many advantageous growth characteristics such a high leaf yield, drought tolerance, adaptation to acidic infertile soils and large temperature variations. However, the widespread implementation of supplementation practices with A. angustissima is currently impeded by reports of anti-nutritive/toxic effects in feeding trials with both monogastric and ruminant animals. Although the plant contains an array of secondary metabolites in high concentrations there have been few documented reports of toxicity under conventional feeding. The intent of this paper is to review the current knowledge on the secondary plant metabolites in A. angustissima, their toxic and anti-nutritive effects and investigations of rumen microbial interactions with these compounds. Condensed tannins, simple phenolics and non-protein amino acids have been investigated experimentally for their role in the reported toxicity of A. angustissima but recent studies have not been able to establish a definitive link between these classes of compounds and toxicity. Attempts to reproduce toxicity in the ruminant animal have also proved difficult particularly when the plant comprizes less than 300 g/kg of the dry matter intake. Ruminal adaptation to the plant occurs in that the major non-protein amino acids are degraded by rumen microorganisms while the condensed tannins promote populations of organisms which are tolerant to these compounds. In vitro studies indicate that condensed tannins in the plant could severely impair nitrogen digestibilility whereas digestibility trials in ruminants show favourable responses in nitrogen utilization when A. angustissima is provided as a supplement. Further research needs to focus on feeding trials in ruminants which demonstrate the nutritional benefits and toxic potential of this plant at varying inclusion rates in the diet.