In vitro ruminal degradation of carbohydrate fractions in tropical grasses fertilized with nitrogen

Abstract

Our goal was to determine the digestion rates of carbohydrate fractions A (sugars, oligosaccharides and organic acids), B1 (starch and soluble fiber), NSC (non-structural carbohydrates) and B2 (available NDF) in four tropical grasses using the gas production technique. Samples of whole forage (WF), residue insoluble in 90% ethanol (EIR) and isolated NDF (ND) were fermented in vitro and gas production measured. Gas volumes determined from the following fractions, A = WF minus EIR; B1 = EIR - ND; NSC = WF - ND; and B2 = ND.  Grasses were Andropogon gayanus, Urochloa brizantha, Cynodon plectostachyus, and Megathyrsus maximum each grown in Veracruz, Mexico on four plots (5×5 m), fertilized (relationship equivalent to 0 and 100 kg N/ha) and clipped 35 d after the N fertilization. A complete randomized block design with factorial arrangement 4×2 and two replicates per treatment was used. Factors were grass species and N fertilization. Data were fit using a single-pool exponential model with lag. The volume (mL gas/100 mg OM), rate (%/h) and lag (h) were: WF (22.8; 5.3; 2.1); A (3.2; 15.7; 0.5); B1 (1.5; 15.7; 0.2); and B2 (18.3; 6.6; 5.2). Andropogon and Urochloa had higher NSC content compared to Megathyrsus and Cynodon but lower gas yield per unit of NSC. Rates of digestion for the B2 fraction ranged from 4 to 8 %/h; and NSC digestion rate averaged 15.7 %/h. Nitrogen fertilization reduced carbohydrate pool sizes but did not affect rates of digestion. It is concluded that ruminally available energy from SC and NSC in tropical forages should be revised more extensively.