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1 Department of Animal and Avian Sciences, University of Maryland, College Park, MD 20742
2 Bovine Genomic Laboratory, Animal and Natural Resources Institute, USDA-ARS, Beltsville, MD 20705
* To whom correspondence should be addressed. E-mail: bbequett{at}umd.edu.
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Abstract |
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The aims were to: 1) determine whether transfer of blood urea to the digestive tract (GIT) or the efficiency of capture of urea-N within the GIT is more limiting for urea-N salvage and 2) establish the relationship between plasma urea concentration and recycling of urea-N to the GIT. We employed an i.v. urea infusion model in sheep to elevate urea entry rate and plasma concentrations, thus avoiding direct manipulation of the rumen environment that otherwise occurs when feeding additional N. Four growing sheep (28.1 ± 0.6 kg) were fed a low protein (6.8% CP, DM basis) diet and assigned to 4 rates of i.v. urea infusion (0, 3.8, 7.5, 11.3 g urea-N/d; 10-d periods) according to a balanced 4 x 4 Latin square design. Nitrogen retention (d 6 to 9), urea kinetics ([15N2]urea infusion last 80-h), and plasma AA were determined. Urea infusion increased apparent total tract digestibility of N (29.9 to 41.3%) and DM (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 increased with plasma urea concentration, but increases were smaller at greater concentrations of plasma urea. Anabolic use of urea-N within the GIT also increased (1.43 to 2.98 g/d; P = 0.003), but anabolic use as a proportion of GIT entry was low and decreased (35 to 22%; P = 0.003) with urea infusions. Consequently, much (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 suggest that transfer of blood urea to the GIT is: 1) highly related to blood urea concentration and 2) less limiting for N retention than is the efficiency of capture of recycled urea-N by microbes within the GIT.
Key Words: digestive tract, nitrogen metabolism, ruminants, urea kinetics
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