Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on nitrogen retention, apparent digestibility, and nutrient flux across visceral tissues in lambs fed low-quality forage

¹ Department of Animal Science, University of Wyoming, Laramie 82071
² Department of Statistics, University of Wyoming, Laramie 82071
³ Department of Animal Science, University of Kentucky, Lexington 40546

^* To whom correspondence should be addressed. E-mail: ludden{at}uwyo.edu.

	Abstract

Two experiments were conducted to determine effects of supplemental ruminally degradable protein (RDP) versus increasing amounts of supplemental ruminally undegradable protein (RUP) on intake, apparent digestibility, N retention, and nutrient flux across visceral tissues in lambs fed low-quality forage. Lambs were fed a basal diet of crested wheatgrass hay (4.2% CP) for ad libitum consumption, plus 1 of 4 protein supplements: isolated soy protein (RDP source) fed to meet estimated RDP requirements (CON), or corn gluten meal (RUP source) fed at 50, 100, or 150% of the supplemental N provided by CON (C50, C100, and C150, respectively). In Exp. 1, 12 lambs (29.9 ± 2.7 kg) were used. Forage OM intake was not affected (P = 0.46) by protein degradability or by increasing RUP (P 0.31). Apparent total tract OM digestibility was not affected (P = 0.10) by protein degradability, but increased (P 0.004) with increasing RUP. Urinary N excretion was not affected (P = 0.20) by protein degradability, but increased (P 0.006) with increasing RUP. Similarly, N retention (g/d) was not affected (P = 0.69) by protein degradability, but increased (P = 0.001) as RUP increased. However, N retention (% of digested N) was not affected (P 0.40) by protein degradability or level of RUP. In Exp. 2, 16 catheterized lambs (32 ± 5 kg) were used. Net release of ammonia-N from the portal-drained viscera (PDV) was greater (P = 0.02) for CON than for C100 and increased linearly (P = 0.002) as RUP increased. Net uptake of ammonia-N by liver was not affected (P = 0.23) by protein degradability, but increased linearly (P = 0.04) as RUP increased. Net urea-N release from liver was not affected (P 0.49) by protein degradability or level of RUP. Net uptake of urea-N by PDV was greater (P = 0.02) for C100 compared with CON and increased (P = 0.04) with increasing RUP. Neither net release from PDV nor hepatic uptake of -amino N were affected (P 0.12) by protein degradability or level of RUP. Hepatic ammonia-N uptake accounted for 82, 38, 98, and 79% of net urea-N release from the liver for CON, C50, C100, and C150, respectively. Hepatic -amino N uptake for all treatments greatly exceeded that required for the remaining urea-N release by the liver, suggesting that -amino N may serve as a temporary means of storing excess N by liver between supplementation events. The pattern of net release or uptake of N metabolites between supplementation events requires further investigation.

Key Words: growing lambs, nitrogen retention, nutrient flux, ruminally undegradable protein

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				R. L. Atkinson, C. D. Toone, and P. A. Ludden Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on site and extent of digestion and ruminal characteristics in lambs fed low-quality forage J Anim Sci, December 1, 2007; 85(12): 3322 - 3330. [Abstract] [Full Text] [PDF]