Portal-Drained Viscera, Hepatic and Lower Hindquarter Net Flux of Nitrogen and Energy Metabolites in Ruminants Fed Protein Supplements Containing Increasing Amounts of Bound Nitrogen and Structural Carbohydrates

Fifteen experiments were conducted on nine mature, nonlactating, nonpregnant ewes to compare distillers dried grains with solubles (DDGS) to soybean meal (SBM) on both an energy and a protein basis. Ewes were equipped with chronic indwelling catheters in the caudal aorta and caudal vena cava, portal, hepatic and mesenteric veins. Ewes were assigned randomly to one of three isocaloric, isonitrogenous diets. Diets were corn cob based and balanced for energy with the addition of corn starch. Protein source for these diets was either SBM, DDGS or a combination of the two, mixed so that 50% of the protein was supplied by DDGS and the remaining 50% supplied by SBM (DDGS + SBM). Animals were fed once daily and consumed a minimum of 1.1 kg/d for a minimum of 10 days prior to blood collection periods. Paraaminohippurate (PAH) was infused into a mesenteric vein and 1 h was allowed for near equilibration. Subsequently, six samples were taken simultaneously from caudal aorta, caudal vena cava, hepatic and portal veins, at 30 min intervals. Following the sixth sample the infusion line was changed to the caudal aorta. A 15 min equilibration time was followed by three samples taken at 15 min intervals, simultaneously from the caudal vena cava and a jugular vein. These samples were used to calculate blood flows across the hindquarters. Samples underwent sameday analysis for glucose, free fatty acids (FFA), PAH (for blood flow), beta-hydroxybutyrate (BOHB) and acetoacetate (ACAC). Plasma was harvested and frozen for later analyses of VFA, ammonia nitrogen (AN), blood urea nitrogen (BUN) and alpha-amino N. Portal drained viscera (PDV) values in kcal/d were similar across diets, indicating similar energy availability from each of the three diets. FFA arterial concentrations were elevated (P<.01) when ewes received DDGS + SBM as a result of a 4 - 5 fold increase (P<.01) in hindquarter release of FFA. No physiologically significant changes occurred with respect to either ACAC of BOHB. A decrease in the percentage of energy released into the portal blood in the form of VFA was noted in DDGS and DDGS + SBM vs. SBM. PDV release of AN was less (P<.05) on DEDGS (8.7 un/h) than SBM (13.8 um/h), while DDGS + SBM was not significantly different than either treatment. Release of BUN into the portal blood was higher (P<.1) on DDGS than SBM, however, liver uptake and calculated TSP release of BUN was similar across diets. Gut release of alpha-amino N was similar across treatments (SBM 8.4 um/h, DDGS 8.4 um/h, DDGS + SBM 13.2 um/h). Based on data reported in this study it seems that DDGS is at least equal to S"BM as a protein supplement, in terms of energy and nitrogen release by the gut and subsequent hepatic and peripheral tissue metabolism.