The objective wasto evaluate in vitro degradation of neutral detergent fiber (NDF) oflow quality tropical forage according to supplementation with nitrogenous compoundsat different true protein:non-protein nitrogen (NPN) ratios. A sample of signalgrass harvested at dry season was used as basal forage. Casein and the mixtureurea:ammonium sulfate (U:AS, 9:1) were used as true protein and NPN source,respectively. The basal supplement for the other was defined by adding caseinthe incubation medium, in order to raise crude protein (CP) level of the basalforage up to 8%, on dry matter basis. The other supplements were defined fromthe fractional replacement (0, 1/3, 2/3 and 1) of casein CP by U:AS proteinequivalents. A control treatment (forage without supplementation) was also evaluated.The treatments were evaluated by in vitro simulated ruminal environment,following the incubation times: 0, 3, 6, 9, 12, 24, 36, 48, 72, and 96 hours.The procedure was repeated three times, totaling three evaluations by incubationtime for each treatment. Incubation residues were evaluated for NDF contentsand interpreted through a non-linear logistic model. Protein supplementationincreased 56.8 to 96.0% the degradation rate of potentially degradable NDF (kpdNDF)in relation to control and decreased discret lag around 4.5 to 7.4 hours. Theexclusive supplementation with urea increased by 15.9% the kpdNDF estimatescompared to exclusive supplementation with casein. It was observed maximum kpdNDFand microbial growth at 2/3 CP of U:AS:1/3 CP of casein. Supplements which provide1/3 of CP protein from true proteinand 2/3 from non-protein nitrogen can optimizethe degradation of NDF of low-quality forage.
