The objective of this work was to evaluate the in vitro degradation dynamics of neutral detergent fiber (NDF) of high-quality tropical forage according to supplementation with different nitrogenous compounds sources. A signal grass sample harvested during rainy season was used as basal forage. Casein was used as true protein source and urea:ammonium sulfate (U:AS, 9:1) mixture (urea) as non-protein nitrogen source. The first three supplements were the addition of 0.5, 1.0 and 2.0 mg of casein per mL of final incubation solution. The urea was added to the same amount of protein supplied by equivalent levels 0.5, 1.0 and 2.0 mg/mL of casein. In total, seven treatments were evaluated, including a control (only grass). The supplements were evaluated in a simulated rumen environment by in vitro incubation, and submitted to different incubation times: 0, 3, 6, 9, 12, 24, 36, 48, 72, and 96 hours. The procedure was repeated four times, making a total of four evaluations per incubation time for each treatment. The incubation residues were evaluated for NDF contents and interpreted through a non-linear logistic model. Casein supplementation of 0.5 mg/mL increased the degradation rate of potentially digestible NDF (kpdNDF) by 1.1%. The addition of 1.0 and 2.0 mg/mL of casein provided an inhibitory effect on the estimates of this parameter in comparison to the control (-6.4 and -9.1% respectively). Moreover, the addition of urea, regardless the supplementation level, increased kpdNDF around 7.6%. The ammonia nitrogen (AN) concentration in the medium was increased by supplementation. However, supplementation with urea, even based on the same crude protein levels used with the supplementation with casein, has given higher ammonia nitrogen values in the incubation medium.
ammonia nitrogen; casein; signal grass; supplement; urea
