Urea recycling contributes to nitrogen retention in calves fed milk replacer and low-protein solid feed

Urea recycling, with urea originating from catabolism of amino acids and hepatic detoxification of ammonia, is particularly relevant for ruminant animals, in which microbial protein contributes substantially to the metabolizable protein supply. However, the quantitative contribution of urea recyclin...

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Bibliographic Details
Published in:	The Journal of nutrition Vol. 144; no. 7; p. 1043
Main Authors:	Berends, Harma, van den Borne, Joost J G C, Røjen, Betina A, van Baal, Jürgen, Gerrits, Walter J J
Format:	Journal Article
Language:	English
Published:	United States 01-07-2014
Subjects:	Animals Animals, Inbred Strains Aquaporin 3 - genetics Aquaporin 3 - metabolism Cattle - growth & development Cattle - metabolism Diet, Protein-Restricted - veterinary Dietary Proteins - administration & dosage Dietary Proteins - metabolism Energy Intake Feces - chemistry Gastric Mucosa - growth & development Gastric Mucosa - metabolism Gene Expression Regulation, Developmental Male Membrane Transport Proteins - genetics Membrane Transport Proteins - metabolism Milk Proteins - metabolism Netherlands Nitrogen - analysis Nitrogen - metabolism Nitrogen - urine Nitrogen Isotopes Plant Proteins - administration & dosage Plant Proteins - metabolism Rumen - growth & development Rumen - metabolism Urea - analysis Urea - metabolism Urea - urine Urea Transporters Weaning
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Summary:	Urea recycling, with urea originating from catabolism of amino acids and hepatic detoxification of ammonia, is particularly relevant for ruminant animals, in which microbial protein contributes substantially to the metabolizable protein supply. However, the quantitative contribution of urea recycling to protein anabolism in calves during the transition from preruminants (milk-fed calves) to ruminants [solid feed (SF)-fed calves] is unknown. The aim of this study was to quantify urea recycling in milk-fed calves when provided with low-protein SF. Forty-eight calves [164 ± 1.6 kg body weight (BW)] were assigned to 1 of 4 SF levels [0, 9, 18, and 27 g of dry matter (DM) SF · kg BW(-0.75) · d⁻¹] provided in addition to an identical amount of milk replacer. Urea recycling was quantified after a 24-h intravenous infusion of [¹⁵N₂]urea by analyzing urea isotopomers in 68-h fecal and urinary collections. Real-time qPCR was used to measure gene expression levels of bovine urea transporter B (bUTB) and aquaglyceroporin-3 and aquaglyceroporin-7 in rumen wall tissues. For every incremental gram of DM SF intake (g DM · kg(0.75)), nitrogen intake increased by 0.70 g, and nitrogen retention increased by 0.55 g (P < 0.01). Of this increase in nitrogen retention, 19% could be directly explained by urea recycling. Additionally, part of the observed increase in nitrogen retention could be explained by the extra protein provided by the SF and likely by a greater efficiency of postabsorptive use of nitrogen for gain. Ruminal bUTB abundance increased (P < 0.01) with SF provision. Aquaglyceroporin-3 expression increased (P < 0.01) with SF intake, but aquaglyceroporin-7 expression did not. We conclude that in addition to the increase in digested nitrogen, urea recycling contributes to the observed increase in nitrogen retention with increasing SF intake in milk-fed calves. Furthermore, ruminal bUTB and aquaglyceroporin-3 expression are upregulated with SF intake, which might be associated with urea recycling.
ISSN:	1541-6100
DOI:	10.3945/jn.114.191353