Ileal Antibiotic Administration and Dietary Nitrogen Level for Beef Heifers Fed High Concentrate Diets

Crude protein requirements of finishing cattle over 1000 lb are below 10 percent of dietary dry matter (NRC, 1976). However, crude protein is usually fed in excess of 10 percent of DM due to decreased feed intake and performance at lower protein levels. Finishing diets usually consist of cereal grains fed ad libitum. With milo and corn grains, substantial quantities of starch are not completely fermented in the rumen and become available in the cecum and proximal colon for further microbial fermentation. Postruminal fermentation may increase the diffusion of urea from blood into the hindgut to supply nitrogen for microbes (Orskov et a1., 1970). Nitrogen uptake by microbes and microbial cell excretion in feces lowers apparent nitrogen digestibility. However, urinary nitrogen excretion may decrease to compensate for elevated fecal loss. Besides N flux, the relationships between hindgut fermentation and ruminal digestion remain unclear. The objectives of this experiment were to investigate effects of antibiotic administration at the ileum and dietary nitrogen level on passage rates and site of digestion in heifers fed high concentrate diets.