Net nutrient flux by visceral tissues of beef steers given abomasal and ruminal infusions of casein and starch.

The objective of this study was to quantify changes of net nutrient metabolism by portal-drained viscera (PDV) and liver of four beef steers (253 +/- 7 kg) in response to combinations of ruminal (R) or abomasal (A) infusions of cornstarch (S) and casein (C). The four treatments in a Latin square design were SACA, SACR, SRCA, and SRCR. Steers were fed alfalfa hay (DMI = 4 kg/d) as a basal diet in 12 equal meals delivered every 2 h and they received continuous infusion of S (800 g/d) and C (200 g/d) in 11-d periods. Digestibilities of DM, N, NDF, and starch, ruminal outflow of liquids and DM, and energy and N retention were less (P < .05) for SA than for SR. Net ammonia and glucose release from PDV were greater (P < .01) for SA than for SR. Net total VFA, acetate, and beta-hydroxybutyrate release from PDV and total splanchnic acetate and beta-hydroxybutyrate release were greater (P < .05) for SR than for SA, but starch infusion site had no effect (P > .10) on net urea N transfer or alpha-amino N release by PDV. Net release of ammonia N, propionate, and total VFA by PDV and uptake of urea N by PDV were greater for CR than for CA, but net alpha-amino N release by PDV and total splanchnic tissues were greater for CA than for CR (P < .05). Summation of energy supply by PDV indicated no difference in total supply among the treatments, but relative contribution of energy sources was affected by infusion site. Energy release by PDV per unit of DE intake was .68 and .66 for SA and SR, respectively. Net release of glucose by PDV was greater for SACA than for SACR (P < .05). These results suggest that site of digestion of starch and casein varies the relative contribution of nutrients to energy supply by visceral tissues and therefore varies N use in beef steers.