Effect of Dietary Protein Level and Protein Source on Plasma and Tissue Free Amino Acids in Growing Sheep

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Abstract
Sheep were used in a 58-day feeding trial to study the influence of a low protein (P) basal ration (6% P) or the basal ration supplemented with soybean meal (12% P), corn protein (17% P), urea (17% P) and fish protein (17% P) on growth and plasma, liver and muscle free amino acids. Plasma amino acids were determined for days 0, 14, 30 and 58 while liver and muscle free amino acids were determined for days 0 and 58 of the trial. Sheep fed the basal ration lost 7.1 g per day while sheep fed the protein-supplemented rations gained between 190 and 230 g per day. There were no gain differences between the P-supplemented rations. In both the plasma and tissue pools, the feeding of the corn protein diet resulted in lowered lysine and elevated leucine levels. Sheep fed the basal and urea-supplemented rations had elevated plasma nonessential to essential amino acids ratios (N/E) and lowered levels of branched-chain amino acids (Br-AA) on day 14, but such trends were not seen for these rations on days 30 and 58. Muscle and liver of sheep fed the basal ration had depressed essential amino acid and Br-AA levels and elevated N/E ratios. At the end of the feeding trial there were few differences in the plasma amino acid patterns of the sheep fed the various rations. These results showed that when a dietary P source bypasses the rumen (i. e., corn protein) its amino acid balance is somewhat reflected in the plasma and tissue pools. These results also suggest that changes in plasma amino acid parameters (i. e., N/E; Br-AA) in response to various rations in ruminants are related primarily to the quantity of protein that reaches the small intestine and are not related to dietary protein source per se.

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