AMINO-ACID AND ENERGY INTERRELATIONSHIPS IN GROWING BEEF STEERS .1. THE EFFECT OF LEVEL OF FEED-INTAKE ON RUMINAL CHARACTERISTICS AND INTESTINAL AMINO-ACID FLOWS

Five cannulated Holstein steers (538 +/- 35 kg) were used in a 4 x 4 L atin square design experiment with extra observations to examine the i nfluence of level of feed intake on postruminal flow and intestinal di sappearance of N and amino acids (AA). Treatments consisted of a singl e diet fed at four levels of energy intake (1.5, 2.0, 2.5, and 3.0 tim es NE, requirement). The diet was formulated on a DM basis to contain 13.25% CP using cracked corn (56.1%), soybean hulls (18%), cottonseed hulls (15%), soybean oil (4.25%), and corn gluten meal (5.6%). Increas ing feed intake linearly increased (P < .0001) the quantity of OM trul y digested in the stomach but tended to decrease (P = .11) OM digestio n as a percentage of intake. Level of feed intake had no effect(P > .1 0) on ruminal pH, NH3 N, or peptide concentration or on particulate an d fluid passage rates. However, total VFA concentration increased line arly (P < .0001) and the acetate: propionate ratio decreased linearly( P < .0001) as feed intake increased. Flows of microbial and nonmicrobi al N at the duodenum linearly increased(P < .002) with increasing inta ke but did not differ (P > .10) as a percentage of intake. Level of fe ed intake did not affect (P > .10) microbial efficiency, N disappearan ce from the small intestine, or total tract N digestibility. With the exception of tryptophan, flows of all individual AA increased linearly (P < .01) with increasing intake. As a percentage of duodenal flow, AA digestion in the small intestine did not differ (P > .10), leading to a linear increase (P < .10) in the net quantity of individual (with t he exception of tryptophan) and total AA disappearing from the small i ntestine as feed intake increased. Likewise, the profile of AA (except tryptophan) disappearing from the small intestine was unaffected (P > .10) by level of feed intake. When compared with predicted requiremen ts for 227-kg growing beef steer, Arg, Met, His, and Lys were suggeste d to be the most limiting AA for growth when this diet is fed. We conc lude that altering energy intake by restricting intake of a single die t has only minor effects on the profile of digestible AA or other nutr ients presented to the animal.