Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: March 11, 2002
Publication Date: July 22, 2002
Citation: Swanson, K., Freetly, H.C., Ferrell, C.L. 2002. Urea-nitrogen recycling and nitrogen balance in lambs fed a high-concentrate diet and infused with differing proportions of casein in the rumen and abomasum [abstract]. Journal of Animal Science. 80(Suppl. 1):72. Technical Abstract: Twenty-five wether lambs (34 +/- 0.9 kg) fitted with ruminal and abomasal infusion catheters were used in a completely randomized design to determine the effects of differing proportions of ruminal and abomasal casein infusion on urea-N recycling and N balance in lambs fed a high-concentrate diet (1.6% N, DM basis) for ad libitum intake. Wethers were infused with 0 (control) or 10.4 g/d of N from casein with ratios of ruminal:abomasal infusion of 100:0, 67:33, 33:67, or 0:100%, respectively, over a 14-d period. Over the last 4 d, urea kinetics were examined by continuous infusion of [15N15N]-urea into the jugular vein and measurement of [15N15N]-urea enrichment in urine using gas chromatography/mass spectrometry. Feed, orts, feces, and urine were collected over the last 5 d. Total nitrogen intake, N excretion, urea-N production, and urea-N recycled to the gastrointestinal tract was greater (P < 0.01) in lambs infused with casein as compared to controls. Nitrogen retention, however, did not differ (P = 0.66; 6.9, 5.5, 7.6, 9.0, and 7.2 g/d; pooled SEM = 0.9) in lambs infused with casein as compared to controls, suggesting N requirements were met without casein supplementation. Total nitrogen intake (24.7, 25.3, 26.9, and 25.1 g/d; pooled SEM = 1.3) and total N excretion (19.2, 17.7, 17.9, and 17.9 g/d; pooled SEM = 1.2) did not differ (P > 0.10) between casein infusion treatments. Urinary N excretion decreased linearly (P = 0.07; 14.2, 12.6, 12.1, and 11.7 g/d; pooled SEM = 0.9) with decreasing ruminal infusion of casein. Urea-N recycled to the gastrointestinal tract increased (P = 0.01; 16.8, 17.2, 22.6, and 23.1 g/d; pooled SEM = 2.0) with decreasing ruminal infusion of casein. These data indicate that decreasing the rumen degradability of supplemental protein, above that required to maximize N retention, results in decreased urinary excretion of N and increased urea-N recycling to the gastrointestinal tract.