Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/6/2000
Publication Date: 5/1/2001
Citation: Ferrell, C.L., Freetly, H.C., Goetsch, A.L., Kreikemeier, K.K. 2001. The effect of dietary nitrogen and protein on feed intake, nutrient digestibility and nitrogen flux across the portal-drained viscera and liver of sheep fed high-concentrate diets ad libitum. Journal of Animal Science. 79:1322-1328. Interpretive Summary: Ruminant animals have the ability to subsist and produce without a source of dietary protein due to the synthesis of microbial protein in the rumen. Microbial protein together with dietary protein, which escapes degradation in the rumen, provide protein (amino acids) to the animal. Ruminant diets frequently require nitrogen or protein supplementation for the animal to perform well. Potential sources of nitrogen differ substantially in cost. Generally, nonprotein nitrogen sources are least, plant sources are intermediate and animal sources of protein are most expensive. The objective of this study was to determine the influence of dietary source of supplemental nitrogen on feed intake, diet digestibility, and post absorptive nitrogen metabolism of sheep fed a high-concentrate diet. Sources of nitrogen evaluated include urea (nonprotein nitrogen that is rapidly degraded in the rumen), soybean meal (source of plant protein that is primarily degraded in the rumen), and a 50:50 mix of blood and feather meals (RUP; animal protein that is poorly degraded in the rumen). Adding urea or soybean meal to a high-grain diet fed to sheep resulted in small increases in feed intake and amino acid uptake from the digestive tract. Sheep consumed 15% less feed, but amino acid uptake was increased 28% when RUP was fed. Net amino acid release from the liver did not differ among dietary treatments, however. Findings indicate there is little advantage to supplementing high-concentrate diets with higher cost protein sources.
Technical Abstract: Objectives were to determine influences of supplemental nonprotein nitrogen or protein on feed intake, digestibility and postabsorptive N metabolism in sheep fed a high concentrate diet ad libitum. Nine Romanov sired, crossbred wethers were fitted with catheters in a mesenteric artery, mesenteric vein, portal vein, and hepatic vein. Treatments consisted of control (no supplemental N; 6.6% CP), or supplemental urea (11.4% CP), soybean meal (SBM; 11.2% CP), or ruminally undegradable protein (RUP; 11.2% CP; 50:50 blood meal and feather meal). Intake or apparently digested DM, OM, and energy did not differ between control and N, or between urea and protein, but were greater in SBM than in RUP supplemented wethers. Intake and apparently digested N was less in wethers fed the control diet than in those receiving N supplementation, but less in RUP than in SBM supplemented wethers. Net portal release and hepatic uptake of alpha-amino N and ammonia N, as well as hepatic release of urea N were greater in wethers supplemented with N than in controls, but portal-drained viscera (PDV) uptake of urea N did not differ among diets. Splanchnic release of alpha- amino N and ammonia N did not differ from 0 nor among diets, but net release of urea N was less for control than for sheep receiving N supplementation. No differences in blood concentration within vessel or net flux across PDV, hepatic or splanchnic tissues of alpha-amino N, ammonia N or urea N were observed among wethers receiving supplemental N. Hepatic oxygen uptake was less in control and urea supplemented sheep than in sheep receiving SBM or RUP. These observations suggest that source of supplemental N had no large effects on the overall N economy of the animals used in this study.