Ryegrass Versus Corn Starch or Beet Pulp Fiber Diet Effects on Digestion and Intestinal Amino Acids in Dairy Cows

In regions with a temperate climate, high yields of grass can be achieved by high N fertilization rates (13). Under these conditions, the CP content of grass is high, but less than 30% of the ingested grass protein reaches the duodenum (1, 23). Consequently, a relatively ABSTRACT . C~anges in digestion and AA supply in darry cows were studied when fresh grass was partly replaced by concentrate mixtures based either on com starch or sugar beet pulp fiber. Treatments were tested in a Latin square utilizing three lactating cows with ruminal and intestinal cannulas. Partial replacement of grass decreased CP digestibility. When high starch concentrate was fed, NoF digestibility was lower than that of the high fiber diet, mainly because of decreased ruminal digestion of NDF. For the high starch concentrate, 39% of the ingested starch escaped ruminal fermentation. Although less OM was fermented in the forestomachs on high starch concentrate, the duodenal AA N flow was higher than for the high fiber concentrate. The proportion of microbial protein was unaffected; thus, efficiency of microbial synthesis was estimated to be higher when high starch concentrate was fed.