In vitro digestibility and chemical composition of three tropical pasture legumes, Desmodium intortum cv. Greenleaf, D. tortuosum and Macroptilium atropurpureum cv. Siratro

Abstract

Leaflets and petioles of Desmodium intortum cv. Greenleaf and Desmodium tortuosum (Florida beggarweed) and leaflets of Macroptilium atropurpureum (Siratro) were analysed for nitrogen: sodium, potassium, phosphorus, carbohydrates (monosaccharides, cyclitols and polysaccharides), organic acids, lignin and polyphenols. The major cyclitol, D(+)-pinitol, was isolated and characterized, along with D-glucose, D-galactose and myo-inositol. The major organid acid was malonic acid, and small amounts of oxalic and succinic acids were also present. The major chemical difference between the legumes appeared in the polyphenols. D. intortum leaflets containqi large amounts of proanthocyanidins (20% catechin equivalent), such polyphenols being undetected in the other legumes. The proanthocyanidins gave three anthocyanidins, two of which were identified (delphinidin and cyanidin). The polyphenols of D. intortum, because of their ability to complex with protein, appeared to be the major cause of the resistance of this legume to bacterial decomposition. However, they did not fully explain the low in vitro digestibility of this legume, since there w'as no significant inhibition of bacterial or pepsin activities, up to 70% of the total crude protein in D. intortum leaflet being liberated with pepsin treatment. The major resistance to in vitro digestion occurred, in the cell wall, where it is possible that some proanthocyanidins were complexed with either cellulose or protein, protecting linkages from cleavage.