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Title: Salvage of blood urea nitrogen in sheep is highly dependent on plasma urea concentration and the efficiency of capture within the diegestive tract

item Baldwin, Ransom - Randy

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2006
Publication Date: 4/1/2007
Citation: Sunny, N.E., Owens, S.L., Baldwin, R.L., El-Kadi, S.W., Kohn, R.A., Bequette, B.J. 2007. Salvage of blood urea nitrogen in sheep is highly dependent on plasma urea concentration and the efficiency of capture within the diegestive tract. Journal of Animal Science. 85(4):1006-1013.

Interpretive Summary: In ruminants, many common feeding conditions lead to Liver urea-N production exceeding apparent digestible N intake (4, 5, 6) and still achieve positive N balance by maintaining high rates of urea-N transfer to and use within the rumen. Thus, understanding the mechanisms involved in the N recycling process and using this knowledge to promote N salvage would improve N efficiency of ruminants with a commensurate reduction in N in manure. The ability of ruminants to partition urea-N to the GIT is much greater and places less a limitation on salvage of N than fermentation events (e.g. pH, CO2, butyrate, ammonia vs. ammonium, soluble carbohydrates) and other physiological processes (e.g. blood vs. saliva transfer, rumen motility and passage rate) occurring within the rumen environment. One proviso to this conclusion, however, is that our measurements were of urea transfer to the whole GIT. Thus, urea hydrolysis probably occurred all throughout the GIT with capture for microbial protein synthesis and amino acid absorption only possible from blood urea-N transferred directly to the rumen. The next advances in efficiency of use of urea-N recycled to the GIT will be based on knowledge of the GIT compartments to which urea is transferred and use by the microbial ecosystems residing in the GIT.

Technical Abstract: The objective was to establish the relationships between transfer of blood urea-N to the digestive tract (GIT) and utilisation of recycled urea-N within the GIT, and to determine which of these two mechanisms of the urea recycling process places greater limits on N salvage by growing sheep. Four growing sheep were fed a low protein (6.8% crude protein, as-fed) diet and assigned to four levels of intravenous urea infusion (0, 3.8, 7.5, 11.3 g urea-N/d; 10-d periods) according to a balanced 4 ' 4 Latin square design. N retention (days 6-9), urea kinetics ([15N2]urea infusion last 80 h) and plasma amino acids were determined. Urea infusion increased apparent digestibility of N (29.9% to 41.3%) and dry matter (47.5 to 58.9%), and N retention (1.45 to 5.46 g/d). Plasma urea-N entry rate increased (5.1 to 21.8 g/d) with urea infusion as did the amount of urea-N entering the GIT (4.1 to 13.2 g/d). Urea-N transfer to the GIT followed a curvilinear relationship with plasma urea concentration. Anabolic use of urea-N within the GIT also increased (1.43 to 2.98 g/d; P < 0.005), however, anabolic use as a proportion of GIT entry was low and decreased (35% to 22%; P < 0.005) with urea infusions. Consequently, most (44 to 67%) of the urea-N transferred to the GIT returned to the liver for re-synthesis of urea (1.8 to 9.2 g/d; P < 0.05). The present results support the contention that transfer of blood urea to the GIT is less limiting for N retention than is the efficiency of capture of recycled urea-N by microbes in the GIT.