Influence of the novel urease inhibitor N-(n-butyl) thiophosphoric triamide on ruminant nitrogen metabolism: I. In vitro urea kinetics and substrate digestion.

Two in vitro digestion experiments were conducted to evaluate the influence of the novel urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) on in vitro urea kinetics, substrate digestion, and fermentation characteristics. In Exp. 1, in vitro incubations were conducted in 50-mL test tubes con- taining .25 g of ground fescue hay to which 0, 6.5, 13, 26, or 52 mg of NBPT in a buffered ruminal fluid innocu- lum was added. Tubes were incubated in triplicate at 39°C and replicated on consecutive days, with NH3 N and urea concentrations measured at 0, 10, 30, 60, 120, 240, and 360 min. Samples for VFA analysis were col- lected at 6 h, and incubations were continued through 48 h to estimate true digestibility (based on NDF analy- sis). Increasing the dose of NBPT tended (P < .12) to linearly depress the rate of urea hydrolysis and de- creased (P < .0004) subsequent NH3 N formation. Al- though total VFA concentration at 6 h increased lin- early (P < .03), acetate:propionate and estimated true digestibility decreased (P < .01) with increasing NBPT concentration. In Exp. 2, we compared in vitro urea kinetics and digestion of forage-only or mixed forage- grain substrates in response to addition of NBPT. In