The effect of dietary nitrogen and protein on feed intake, nutrient digestibility, and nitrogen flux across the portal-drained viscera and liver of sheep consuming high-concentrate diets ad libitum

Our objectives were to determine the influences of supplemental nonprotein N or protein on feed intake, digestibility, and postabsorptive N metabolism in sheep fed a high-concentrate diet for ad libitum consumption. Nine Roma nov-sired, crossbred wethers (13 mo old; 52 kg) were fitted with catheters in a mesenteric artery, mesenteric vein, portal vein, and hepatic vein. Wet hers consumed a 95% concentrate diet ad libitum. Treatments consisted of co ntrol (no supplemental N; 6.6% CP) or supplemental urea (11.4% CP), soybean meal (SBM; 11.2% CP) or ruminally undegradable protein (BFM;: 11.2% CP; 50 :50 blood meal and feather meal). Intake or apparently digested intake of D M, OM, and energy:did not differ between control and N-supplemented (P > 0. 40), or between urea- and protein-supplemented (P > 0.40), but were greater (P < 0.05) in SBM-than in BFM-supplemented wethers. Intake and apparently digested intake of N were less (P < 0.01) in wethers fed the control diet t han in those receiving N supplementation but were less (P = 0.03) in BFM- t han in SBM-supplemented wethers. Neither portal nor hepatic venous blood fl ows differed (P > 0.15) among treatments. Net portal release and hepatic up take of alpha -amino N and ammonia N and hepatic release of urea N were gre ater (P < 0.05) in wethers supplemented with N than in controls, but portal -drained viscera (PDV) uptake of urea N did not differ (P > 0.40) among die ts. Splanchnic release of alpha -amino N and ammonia N did not differ from 0 or among diets (P > 0.10), but net release of urea N was less (P = 0.05) for control than for sheep receiving N supplementation. No differences (P > 0.10) in blood concentration within vessel or net; flux across PDV, hepati c, or splanchnic tissues of alpha -amino N, ammonia N, or urea N were obser ved among wethers receiving supplemental N. Net uptake of oxygen by the PDV did not differ among diets, but hepatic uptake was less (P < 0.05) in cont rol and urea-supplemented sheep than in sheep receiving SBM or BFM. These o bservations suggest that the source of supplemental N had no large effects on the overall N economy of the animals used in this study.