Effects of barley grain processing and dietary ruminally-degradable protein on urea-nitrogen recycling and nitrogen metabolism in growing lambs

¹ Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Canada S7N 5A8

^* To whom correspondence should be addressed. E-mail: tim.mutsvan{at}usask.ca.

	Abstract

The objective of this study was to determine how interactions between dietary ruminally-degradable protein (RDP) level and ruminally-fermentable carbohydrate (RFC) alter urea-N transfer to the gastrointestinal tract (GIT) and the utilization of this recycled urea-N in rapidly-growing lambs fed high N diets. Four Suffolk ram lambs (34.8 ± 0.5 kg BW) were used in a 4 x 4 Latin square design with 21-d periods and a 2 x 2 factorial arrangement of dietary treatments. The dietary factors studied were: 1) dry-rolled vs. pelleted barley as the principal source of RFC; and 2) dietary levels of RDP of 60 vs. 70% (% of CP). All diets contained 28.8 g N/kg DM. Experimental diets were composed of 80% concentrate mixture and 20% barley silage (DM basis) and were fed twice daily at 0900 and 1700 as total mixed rations. Nitrogen balance was measured from d 15 to d 20, and urea-N kinetics were measured from d 15 to d 19 using intra-jugular infusions of [¹⁵N¹⁵N]-urea. Nitrogen intake (P = 0.001) and fecal (P = 0.002) and urinary (P = 0.03) N excretion increased as dietary RDP level increased, but method of barley processing had no effect. Feeding dry-rolled compared to pelleted barley (P = 0.04) as well as feeding 60% RDP compared to 70% RDP (P = 0.04) resulted in a higher N digestibility. Whole-body N retention was unaffected (P 0.74) by dietary treatment. Dietary treatment had no effect on endogenous production of urea-N and its recycling to the GIT; however, across dietary treatments, endogenous production of urea-N (45.8 to 50.9 g/d) exceeded N intake (42.3 to 47.9 g/d). Across dietary treatments, 30.6 to 38.5 g/d of urea-N were recycled to the GIT, representing 0.67 to 0.74 of endogenous urea-N production; however, 0.64 to 0.76 of urea-N recycled to the GIT was returned to the ornithine cycle. In summary, although dietary treatment did not alter urea-N kinetics, substantial amounts of hepatic urea-N output were recycled to the GIT under the dietary conditions employed in this study, and additional research is required to determine how this recycled urea-N can be efficiently captured by bacteria within the GIT.

Key Words: nitrogen metabolism, ruminally-degradable protein, processed barley, sheep, urea-nitrogen recycling

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