CHEMICAL CONSTRAINTS TO THE USE OF TROPICAL LEGUMES IN ANIMAL NUTRITION

Browse and grain legumes offer considerable potential for enhancing animal productivity in the tropics. However, a major factor limiting the wider use of these plants is the ubiquitous distribution of a diverse array of natural compounds which are capable of precipitating adverse effects in animals. Manifestations of toxicity range from a marked reduction in animal performance and nutrient utilisation to profound neurological effects and increased mortality. This review identifies the primary anti-nutritional factors in tropical legumes and evaluates the prospects for their detoxification. Although protease inhibitors are widely distributed in legume forage and grains, their role appears to be secondary to that of other toxic compounds. Recent studies highlight the dominant impact of lectins in determining the nutritional value of seeds of Psophocarpus tetragonolobus (winged bean) and Canavalia ensiformis (jack bean). The presence of condensed tannins is a characteristic feature of many browse legumes, including Gliricidia sepium, Acacia species, Leucaena leucocephala and Albiziafalcataria. These polyphenolic compounds are sometimes accorded with beneficial attributes in ruminant nutrition by vir-tue of their capacity to suppress bloat and to prevent excessive degradation of high-quality leaf protein in the rumen. However, with respect to the condensed tannins of tropical forage legumes, there is increasing evidence of toxicity in all classes of livestock, including ruminants. The occurrence of toxic non-protein amino acids is a distinguishing feature of many forage and grain legumes. These amino acids contribute significantly to the toxicity of Leucaena leucocephala, Canavalia ensiformis and Lathyrus species. The distribution of several anti-nutritional factors in a single legume species causes difficulties with detoxification, particularly if the deleterious compounds are heat stable. Significant experimental procedures have recently been described for the detoxification of the winged bean and the jack bean, and these methods now await economic appraisal. The treatment of harvested leaves or the spraying of leguminous shrubs with polyethylene glycol (PEG) to alleviate the deleterious effects of condensed tannins is a comparatively recent innovation worthy of further investigation, particularly in view of striking improvements in the performance of sheep fed Acacia aneura. The most notable advance in detoxification relates to biotechnological manipulation of rumen function, enabling cattle grazing Leucaena forage to degrade the deleterious amino acid, mimosine, to innocuous residues. The prospects for exploiting this technique in the detoxification of other tropical legumes and forage plants should now be considered. However, the scope for any significant role of leguminous leaf meals in the nutrition of non-ruminant animals now appears to be limited, despite recent advances in detoxification. In the case of these feeding stuffs, the major constraints are the intrinsic low digestibility of the protein fraction and the inferior metabolisable energy content relative to that of the legume grains.