Ruminal and postruminal digestion of dietary protein and starch in steers: 1. Effects of protein concentration, degradation of protein and energy content of the diet

Different protein-energy ratios in feedlot diets may effect several combinations of amounts of amino acids and glucose available for absorption in the small intestine. ln this regard, the effect of protein concentration and degradation, and energy content of the diet were studied in two 4 x 4 Latin Square trials with ruminal and duodenally fistulated steers. In the first trial, steers were fed diets containing 10.0 MJ ME/kg DM at two protein concentrations (105and 125 g CP/kg DM), each attwo rumen degradation levels (60 and 70% RDP). In the second, the dietary energy contentwas 12.5 MJ ME/kg DM, thetwo protein concentrationswere 135 and 155 g CP/kg DM and the two RDP levels were also 60 and70o/o. Theprotein-energy ratio was the same for both energy levels. All diets were fed at a level of 100 g air dry feed/kg \ P/c/d. An increase in N intake increased NAN passage and apparent absorption in the lower digestive tract. RDP level did not have a significant effect on NAN passage. The percentage OM that apparently fermented in the rumen (OMDR) was negatively associated with NAN passage. OMDR was lower on the higher energy diet; the difference being mainly due to the difference in starch digestion. Proportionally less starch was digested in the rumen and more in the lower digestive tract with the higher energy diet, i.e. the higher starch diet. Results indicate that ratios of amino acids and glucose available for absorption from the small intestine vary independently of the dietary protein-energy ratio.