Effect of Dietary Protein on Amino Acids and Microbial Protein of Duodenal Digesta

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Abstract
Three duodenal reentrant cannulated Holstein cows were fed each of 7 randomly assigned rations. Rations were brome hay, brome hay plus concentrate mix and brome hay plus concentrate with 1 of 5 sources of supplemental protein. Protein sources were urea, linseed meal, soybean meal, brewer''s dried grain and feather meal. Cows were adapted to each treatment ration for 14 days prior to sampling for 3 days. The quantity of total essential and nonessential amino acids in the digesta differed for all rations from most to least: urea, feather meal, brewer''s dried grain, linseed meal, soybean meal, without supplemental protein, hay. Regardless of the source of supplemental protein fed, concentration of total essential amino acids increased and total nonessential amino acids decreased in the digesta compared to their dietary concentration. Threonine, isoleucine, lysine and tyrosine concentrations increased glutamic acid, proline and cystine decreased in the digesta compared to their dietary compositions. Daily dry matter and microbial N passage through the duodenum was greatest for cows fed supplemental urea. With exception of the urea treatment, duodenal passage rates of fluid, dry matter, N and amino acids increased as N solubility of the ration decreased.