Four Corriedale wethers averaging 45 kg of body weight were randomly assigned to a 4 x 4 Latin square to evaluate the effects of two carbohydrate sources (soybean hulls and corn grain) and two dietary forage levels (40 and 70%) on nutrient digestibility, N balance, and portal nutrient flux. Animals were fitted with catheters in the mesenteric and portal veins and in the mesenteric artery. Digestibility of nutrients and N balance were determined by total collection of feces and urine. Portal plasma flow was measured by continuous infusion of P-aminohippurate, and net nutrient flux calculated as the difference between venous and arterial concentration times blood flow. Dry matter digestibility and intake and digestion of ether extract were greater on corn than on soybean hulls diets while the opposite was observed for intake (789.7 g/d), digestion (444.2 g/d), and digestibility (56.2%) of NDF. There was a significant interaction between carbohydrate sources and dietary forage levels for intake and digestion of NFC; corn and lower forage diets resulted in greater intake and digestion of NFC whereas no effect of forage level was observed on soybean hulls diets. Energy concentration expressed as TDN was higher for corn diets. Diets with 40% forage showed improved N utilization because of the reduced excretion of N in urine and feces and increased N digestibility and retention. Portal and arterial glucose concentrations were higher on animals fed corn and 40% forage diets. Portal ammonia and urea concentration, venous-arterial difference and portal ammonia flux were all lower on diets with 40% forage.
corn; digestibility; level of forage; nitrogen balance; portal flow; soybean hulls
