title: 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.
abstract: Three lamb metabolism experiments were conducted to investigate the effects of chronic administration of the novel urease inhibitor N (n-butyl) thiophosphoric triamide (NBPT) on ruminal N metabolism, fermentation, and N balance. In Exp. 1, ruminally cannulated wethers (n = 28; 45.0 +/- .9 kg) were administered one of seven doses of NBPT (0 [control], .125, .25, .5, 1, 2, or 4 g of NBPT daily) and fed a common cracked corn/cottonseed hull-based diet 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, the detection of an NBPT x day interaction (d 2 vs 15; P < .01) indicated that this depression in urea degradation diminished as the experiment progressed. 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. Increasing NBPT had no effect (P > .10) on DM or ADF digestibilities, but it linearly 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 interactions (P > .10) for Exp. 2. Increasing NBPT depressed (linear and quadratic P < .0001) ruminal urease activity, producing linear (P < .0001) increases in urea N and linear decreases in NH3 N in the rumen. As in Exp. 1, an NBPT x day interaction (P < .05) was noted for urea, NH3 N, and total VFA concentrations; the maximum response to NBPT occurred on d 2 but diminished by 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 interactions (P < .05) were noted, increasing NBPT did not affect (P > .10) N digestibility. Administration of NBPT quadratically increased (P < .10) urinary N excretion, producing a linear decrease (P < .05) in N retention. These results suggest that although NBPT is capable of inhibiting ruminal urease short-term, the ruminal microflora may be capable of adapting to chronic NBPT administration, thereby limiting its practical use in improving the utilization of dietary urea.
authors.affiliation: Department of Animal Sciences, University of Kentucky, Lexington 40546, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Ludden
authors.forename: D L
authors.initials: DL
authors.lastname: Harmon
authors.forename: G B
authors.initials: GB
authors.lastname: Huntington
authors.forename: B T
authors.initials: BT
authors.lastname: Larson
authors.forename: D E
authors.initials: DE
authors.lastname: Axe
mesh_terms: D000641:Ammonia / Q000097:blood; D000821:Animal Feed; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D004063:Digestion / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D009584:Nitrogen / Q000378:metabolism*; D009943:Organophosphorus Compounds / Q000494:pharmacology*; D012417:Rumen / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D012756:Sheep / Q000254:growth & development / Q000378:metabolism*; D014510:Urease / Q000037:antagonists & inhibitors*
chemical_list: D001786:Blood Glucose; D004791:Enzyme Inhibitors; C120049:N-(n-butyl) thiophosphoric triamide; D009943:Organophosphorus Compounds; D000641:Ammonia; D014510:Urease; D009584:Nitrogen