The objective was to evaluate in vitro degradation of neutral detergent fiber (NDF) of low quality tropical forage according to supplementation with nitrogenous compounds at different true protein:non-protein nitrogen (NPN) ratios. A sample of signal grass harvested at dry season was used as basal forage. Casein and the mixture urea: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 casein the incubation medium, in order to raise crude protein (CP) level of the basal forage up to 8%, on dry matter basis. The other supplements were defined from the fractional replacement (0, 1/3, 2/3 and 1) of casein CP by U:AS protein equivalents. 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 incubation time for each treatment. Incubation residues were evaluated for NDF contents and interpreted through a non-linear logistic model. Protein supplementation increased 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. The exclusive supplementation with urea increased by 15.9% the kpdNDF estimates compared to exclusive supplementation with casein. It was observed maximum kpdNDF and microbial growth at 2/3 CP of U:AS:1/3 CP of casein. Supplements which provide 1/3 of CP protein from true proteinand 2/3 from non-protein nitrogen can optimize the degradation of NDF of low-quality forage.
ammonia nitrogen; casein; degradation rate; signal grass; urea
