Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2006
Publication Date: 4/1/2007
Citation: Reynal, S.M., Ipharraguerre, I.R., Lineiro, M., Brito, A.F., Broderick, G.A., Clark, J.H. 2007. Omasal flow of soluble proteins, peptides, and free amino acids in dairy cows fed diets supplemented with proteins of varying ruminal degradabilities. Journal of Dairy Science. 90:1887-1903. Interpretive Summary: The dairy cow gets the protein it uses to maintain its body and to produce milk from microorganisms living in the rumen (the first compartment of the cow’s stomach) and protein of dietary origin that escapes microbial digestion in the rumen. Amino acids, which are the building blocks of protein, are found in the rumen as intact proteins plus peptides (protein fragments of various sizes) and as free amino acids. It is not known whether peptides and free amino acids occurring in the rumen make an important contirbution toward meeting the amino acid needs of the cow. The purpose of this experiment was to measure the total amino acids that flow out of the rumen as intact protein versus peptides and free amino acids, and whether these were of microbial versus dietary origin. This study was conducted using lactating dairy cows that were prepared with rumen cannulas (openings into the rumen made surgically by veterinarians). Peptides were measured in the digesta flowing out of the rumen using a new ultra-filtration method that distinguished peptides from proteins, and among peptides of differing sizes, based on molecular weight. Depending on the dietary protein fed, peptides plus a small amount of free amino acids accounted for 9 to 16% of the total flow of amino acids out of the rumen. Source of dietary protein had only a small affect on amino acid flow in peptides of different molecular sizes; however, greater proportions of smaller peptides than larger peptides were contributed by the diet. On average, 10% of the total amino acids flowing out the rumen in peptides were of dietary origin. These results show that ultra-filtration gives a more accurate measurement of the rumen flow of peptide amino acids. Moreover, our findings indicate that a small but significant part of the cow’s requirements for amino acids is met from small peptides that originate from the diet. Until now, dairy scientists have largely or completely ignored the contribution of this source of amino acids toward meeting the nutritional needs of the high producing dairy cow.
Technical Abstract: Three ruminally and duodenally cannulated cows were assigned to an incomplete 4 x 4 Latin square with four 14-day periods and fed diets supplemented with urea, solvent soybean meal (SSBM), xylose-treated soybean meal (XSBM), or corn gluten meal (CGM) to study the effects of crude protein source on omasal canal flows of soluble amino acids (AA). Soluble AA in omasal digesta were fractionated by ultrafiltration into soluble proteins greater than 10 kDa (10K), oligopeptides between 3 and 10 kDa (3-to-10K), peptides smaller than 3 kDa (small peptides), and free AA (FAA). Omasal flow of total soluble AA ranged from 254 to 377 g/d and accounted for 9.2 to 15.9% of total AA flow. Averaged across diets, flows of AA in 10K, 3-10K, small peptides, and FAA were 29, 217, 50, and 5 g/d, respectively, and accounted for 10.3, 71.0, 17.5, and 1.6% of the total soluble AA flow. Cows supplemented with SSBM had higher flows of Met, Val, and total AA associated with small peptides than those supplemented with XSBM, whereas supplementation with CGM resulted in higher total small peptide-AA flows than XSBM. Averaged across diets, 27, 75, and 93% of soluble AA in 10-K, 3-10K, and peptides plus FAA flowing out of the rumen were of dietary origin. On average, 10% of the total AA flow from the rumen was soluble AA from dietary origin, indicating a substantial escape of dietary soluble N from ruminal degradation. Omasal concentrations and flows of soluble small peptides isolated by ultrafiltration were substantially smaller than most published ruminal small peptide concentrations and outflows measured in acid-deproteinized supernatants of digesta. The use of ultrafiltration may allow more accurate quantitation of the ruminal peptide pool, its variations caused by the diet, and its metabolic and nutritional significance to the ruminal microbes and the host animal.