Fibrous Carbohydrate Fractions and in Vitro True and Apparent Digestibility of 10 Tropical Forage Grasses

Abstract

Ten tropical forage grasses (Pangola, Digitaria decumbens; Congo, Brachiaria ruziziensis; Signal, Brachiaria brizantha; Buffel, Cenchrus ciliaris; Guinea, Panicum maximum; Jaragua, Hyparrhenia rufa; Giant Pangola, Digitaria valida; African Crab, Digitaria swazilandensis; Venezuelan Elephant, Pennisetum setosum; and Limpo, Hemarthria altissima) lightly fertilized with NH₄NO₃ at the rate of 350 kg/ha, were harvested by hand (machete) every 30 days up to 180 days of age, beginning August 20, 1970, at the College of Agricultural Sciences grass collection, University of Puerto Rico at Mayagüez. They were evaluated chemically for cell-wall constituents or neutraldetergent fiber (NDF), acid-detergent fiber (ADF), permanganate lignin (L), cellulose (C) and silica (Si), as outlined by Goering and Van Soest and for in vitro true digestibility (IVTD), utilizing the Goering and Van Soest modification of the Tilley and Terry procedure. Hemicellulose (H) was determined as the difference between NDF and ADF. Statistical analyses of variance were conducted. Highly significant differences in NDF, ADF, H, L, L/ADF, C, and Si contents were observed between species. IVTD and in vitro apparent digestibility (IVAD) also revealed highly significant differences between species. Guinea and Buffel grasses exhibited the highest mean values in NDF and ADF contents but the lowest in IVTD and IVAD, suggesting that, as total fiber and lignocellulose contents increase, digestibility decreases. Similar trends were observed in other species. Highest mean values were obtained in Guinea grass for C, in Jaragua grass for Si and in Limpo grass for L and H. As the grasses advanced in maturity from 30 to 180 days, NDF, ADF, L, L/ADF, C and Si revealed highly significant increases while IVTD and IVAD revealed highly significant decreases with HC remaining almost constant throughout. The largest increase in NDF, ADF, L and C contents and the largest decrease in IVTD and IVAD evaluations occurred between the 30- and 60-day intervals, probably due to the high environmental temperature and high transpiration causing early onset of lignification and silicification in tropical climates. Pangola grass underwent little change in nutritive value as compared to other species. Evidence is presented that fibrous fractions and IVTD evaluations of all species compared favorably among themselves and that all fibrous fractions (except H) and IVTD differed significantly with plant age.