Monosaccharides in Detergent Residues of Tropical Forage Plants1

Abstract

Detergent extractions have been widely used to evaluate forage quality. Quantities of the major structural polysaccharides, cellulose and hemicellulose, have been calculated from these empirical fractionations and lignin content. The presence of noncellulose carbohydrate in acid detergent (AD) residue (which theoretically is composed of cellulose and lignin) of temperate forages has been reported by several investigators. Leaf anatomy and cell wall contents of temperate and tropical forage plants differ distinctly. Leaf samples from field‐grown plants of the tropical grasses ‘Ona’ stargrass (Cynodon nlemfuensis Vanderyst) and ‘Pensacola’ bahiagrass (Paspalum notatum Fluegge) and the tropical legumes ‘Siratro’ [Macroptilium atropurpureum (DC.) Urb.] and Vigna adenantha (G.F. Meyer) Marechal, Mascherpa, and Stainier were used to evaluate the neutral monosaccharide compositions of detergent residues of tropical forage plants. Leaf samples were extracted with neutral detergent (ND), AD, sequential ND and AD, and sequential ND and AD with AD reflux time increased to 2 h. Resulting residues were deliguifled with sodium‐chlorite and acid‐hydrolyzed before neutral monosaccharide compositions were determined by gas chromatography of alditoloacetate derivatives. Glucose comprised 875 and 927 g kg−1 of the neutral monosaccharides of AD residues of stargrass and bahiagrass, respectively, with similar results from the sequential extractions. Neutral monosaccharides from AD residues of Siratro and Vigna contained 815 and 807 g of glucose kg−1 of residue, respectively. The only extraction yielding a greater residue glucose concentration than AD alone was the ND, followed by 2‐h AD extraction of Vigna, which increased glucose concentration from 807 to 914 g kg−1. Although AD residues of tropical grasses were similar in neutral monosaccharide composition to published data for temperate grasses, AD residues of tropical legumes contained more neutral monosaccharides derived from polysaccharides other than cellulose. Therefore, detergent extractions of tropical forage plants, especially tropical legumes, do not provide an adequate base for calculation of cellulose and hemicellulose contents.