Influence of the novel urease inhibitor N-(n-butyl) thiophosphoric triamide on ruminant nitrogen metabolism: II. Ruminal nitrogen metabolism, diet digestibility, and nitrogen balance in lambs

Three lamb metabolism experiments were conducted to investigate the effects of chronic administration of the novel urease inhibitor N (n-butyl) thioph osphoric triamide (NBPT) on ruminal N metabolism, fermentation, and N balan ce. In Exp. 1, ruminally cannulated wethers (n = 28; 45.0 +/- .9 kg) were a dministered one of seven doses of NBPT (0 [control], .125, .25, .5, 1, 2, o r 4 g of NBPT daily) and fed a common cracked corn/cottonseed hull-based di et twice daily containing 2% urea at 2.5% of initial BW for the duration of the 15-d experiment. Overall, NBPT decreased (linear P < .0001; quadratic P < .001) ruminal urease activity, resulting in linear increases (P < .0001 ) in ruminal urea and decreases in ruminal NH3 N concentrations. However, t he detection of an NBPT x day interaction (d 2 vs 15; P < .01) indicated th at this depression in urea degradation diminished as the experiment progres sed. Increasing NBPT linearly decreased (P < .01) total VFA concentrations on d 2 of the experiment, but it had no effect (P > .10) on d 15. Increasin g NBPT had no effect (P > .10) on DM or ADF digestibilities, but it linearl y decreased (P < .01) N digestibility. Supplementing NBPT produced a linear increase (P < .05) in urinary N excretion and a linear decrease (P < .01) in N retention. In Exp. 2, ruminally cannulated wethers (n = 30; 46.8 +/- . 6 kg) were fed one of two basal diets (2.0 vs 1.1% dietary urea) at 2.5% of initial BW and dosed with either 0 (control),.25, or 2 g of NBPT daily for the duration of the 15-d experiment. There were no NBPT x dietary urea int eractions (P >.10) for Exp. 2. Increasing NBPT depressed (linear and quadra tic P < .0001) ruminal urease activity, producing linear (P <.0001) increas es in urea N and linear decreases in NH3 N in the rumen. As in Exp. 1, an N BPT x day interaction (P < .05) was noted for urea, NH3 N, and total VFA co ncentrations; the maximum response to NBPT occurred on d 2 but diminished b y d 15 of the experiment. Administration of NBPT did not influence (P >.10) DM, ADF, or N digestibilities in Exp. 2. In Exp. 3, wether lambs (n 30; 26 .4 +/- .7 kg) were subjected to the same treatment regimen as in Exp. 2 for a 14-d N balance experiment. Although several. NBPT x dietary urea interac tions (P < .05) were noted, increasing NBPT did not affect (P >.10) N diges tibility. Administration of NBPT quadratically increased (P < .10) urinary N excretion, producing a linear decrease (P < .05) in N retention. These re sults suggest that although NBPT is capable of inhibiting ruminal urease sh ort-term, the ruminal microflora may be capable of adapting to chronic NBPT administration, thereby limiting its practical use in improving the utiliz ation of dietary urea.