Influence of feed intake level on site of digestion in steers fed a high concentrate diet.

A high concentrate diet was fed at 1.2, 1.5, 1.8 and 2.1% of body weight to four Angus steers (258 kg) equipped with cannulas in the proximal duodenum and distal ileum. As level of feed intake increased, flow of N, nonammonia N, microbial N and feed N to the small intestine increased linearly (P less than .05). Bypass of feed N increased from 44 to 71% of fed N. At the highest intake level, ruminal protein degradation and protein solubility were almost equal. At lower intake levels, feed protein degradation exceeded N use by microbes, but at the higher level of intake, microbial protein synthesis exceeded N available from protein degradation. Microbial efficiency increased as feed intake was increased to 1.8% of body weight. Apparent digestion of N in the small intestine increased with feed intake. Starch digestion in the rumen increased slightly with feed intake while organic matter and acid detergent fiber (ADF) digestibility declined. At the highest level of feed intake, no ADF disappearance occurred in the rumen. Results suggest that level of feed intake markedly alters 1) bypass and supply of intestinally digested protein, 2) need for degradable N in the rumen, 3) efficiency of microbial growth and 4) ruminal fiber digestion. Both retention time and contingent characteristics of fermentation in the rumen appear to be involved in these alterations.