Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2003
Publication Date: 2/1/2004
Citation: SWANSON, K., FREETLY, H.C., FERRELL, C.L. NITROGEN BALANCE IN LAMBS FED A HIGH-CONCENTRATE DIET AND INFUSED WITH DIFFERING PROPORTIONS OF CASEIN IN THE RUMEN AND ABOMASUM. JOURNAL OF ANIMAL SCIENCE. 2004. V. 82. P. 495-501. Interpretive Summary: Because ruminants are inefficient at utilizing dietary nitrogen, there is a high cost of conversion from feed to product and high levels of nitrogen are excreted into the environment. Volatilization of nitrogen from the feedlot surface in the form of ammonia is a large concern because of the environmental consequences and the imbalance it creates in the nitrogen to phosphorous ratio when manure is used as fertilizer. Absorbed dietary nitrogen not utilized by the animal is converted to urea and is either excreted in urine or is recycled to the gastrointestinal tract for reutilization. By increasing the proportion of urea which is recycled to the gastrointestinal tract, nitrogen retention could increase and nitrogen excretion decrease. Protein supplementation of corn-based diets often is necessary for maximal performance. Protein supplements differ in the amount of protein which is degraded by the microflora in the rumen (ruminally degradable intake protein) or bypasses rumen degradation (ruminally undegradable intake protein). However, little is known about how ruminal degradability of supplemental protein influences urea recycling in high concentrate diets and how changes in urea nitrogen recycling influence nitrogen retention and excretion. Therefore, to simulate supplements with different ruminal protein degradability, casein was infused with different proportions (100:0, 67:33, 33:67, and 0:100%) ruminally and post-ruminally. A control treatment in which water was infused both ruminally and post-ruminally was also included. Retention of nitrogen did not differ between the controls and lambs infused with casein partly because a larger proportion of urea production was recycled to the gastrointestinal tract and thus, not excreted in urine. Urinary nitrogen excretion decreased and urea nitrogen recycling increased as casein infusion was shifted post-ruminally. These data emphasize the importance of urea nitrogen recycling in ruminants fed high-concentrate diets. These data also emphasize the importance of meeting, but not exceeding, the ruminally degradable intake protein requirement in growing ruminants fed high concentrate diets as use of excess ruminally degradable intake protein is limited. Additionally, supplementing ruminally undegradable intake protein to lambs fed corn-based diets likely is unnecessary. However, supplementing excess ruminally undegradable intake protein may have less deleterious effects than supplementing excess ruminally degradable intake protein because of the shift in site of nitrogen excretion from urine to feces which could result in a reduction in ammonia volatilization from the feedlot surface.
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 (85% corn grain, 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%, over a 14-d period. Over the last 4 d, urea kinetics were quantified by continuous infusion of [15N15N]-urea into the jugular vein and measurement of [15N15N]- and [14N15N]-urea enrichment in urine using gas chromatography/mass spectrometry. Feed, orts, feces, and urine were collected over the last 5 d. Total N 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. However, N retention did not differ in lambs infused with casein as compared to controls, suggesting N requirements were met without casein supplementation. Total N intake and total N excretion did not differ (P > 0.10) between casein infusion treatments. Urinary N excretion decreased linearly (P = 0.07) with decreasing ruminal infusion of casein. Site of casein infusion quadratically (P = 0.06) influenced N retained (g/d) with the greatest retention observed in the 33R:67A treatment. Urea N recycled to the gastrointestinal tract increased with decreasing ruminal infusion of casein. Dry matter intake from feed decreased from 1,183 to 945 g/d (P = 0.02) in lambs infused with casein as compared to controls, but apparently digested DM did not differ between treatments. These data indicate that urea N recycling is a major source of N within the gastrointestinal tract and 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 without greatly affecting apparent diet digestion.