The nutritional value of tropical legume forages fed to ruminants as affected by their growth habit and fed form: A systematic review

•The nutritional value of legumes strongly varies depending on their growth habit.•Herb legumes have the highest nutritional value, shrubs have the lowest.•The fed form interacts with the nutritional value of herbs, shrubs and trees.•Fiber-bound nitrogen and lignin potentially limit the value of tropical legumes.•Micro minerals, sugars, starch, pectin, amino acids were not found for this review. This systematic review summarized literature data on the nutritional composition of tropical legume forages fed to cattle, sheep and, goats, explaining the nutritional variation between legumes depending on their growth habit (i.e. herbs, shrubs, trees) and the form in which they are fed to the ruminant (i.e. fresh, hay, silage, haulm). One-hundred and eleven legume species were recorded from 399 in vivo studies, reflecting the diversity inherent to the tropics. However, those figures also reflect that, different to temperate legumes, research in tropical forages spreads across a large number of legume species, opposite to focusing on a limited number of promising species. Tree legumes had higher crude protein and lower fiber concentration than herbs and shrubs, likely because leaves and young stems are fed from trees. Herbs had a lower concentration of lignin than both shrub and tree legumes. Lignin concentration was higher in legumes, compared with tropical grasses, particularly for trees and shrubs. Lignin appears to limit in vitro dry matter digestibility and metabolisable energy of legumes, hampering their potential as feed. High nutrients losses appeared when legumes were fed as hay or exclusively legume silage. Fiber-bound nitrogen was greater in shrubs and trees than in grasses, and was increased by the process of hay-making. From the perspective of the nutritional value herb legumes appear to be the most promising as forage materials. Limited information was found for minerals other than Ca and P, and for protein- and carbohydrate-fractions, starch, indigestible-NDF, pectin, or amino acids, indicating future research avenues for a complete evaluation of tropical legumes as feed resources.