Abstract
Free amino acid ratios in rumen fluid, blood plasma, milk, and feces of lactating dairy cows were studied following the feeding of four experimental concentrates: A, basal; B. basal plus .11% DL methionine; C. basal plus .11% methionine hydroxy analog (M-analog); and D. basal plus .22% M-analog. Free threonine was increased in rumen fluid by supplementation with the higher M-analog. No significant changes were detected in the proportions of any of the free amino acids of blood plasma. Methionine and lower M-analog supplementation increased aspartic acid and valine in free amino acids of milk.There was a close similarity between proportions of free lysine, glutamic acid, arginine, proline, tyrosine, and phenylalanine in rumen fluid and in milk. The proportions of other free amino acids in rumen fluid and milk were not similar. There was no similarity between proportions of free amino acids in blood with those in rumen fluid or in milk.Experimental concentrates had no effect on the free amino acid content of feces. There was no significant relationship between free fecal amino acids and the free amino acid content of rumen fluid, blood plasma, or milk. Milk yield, body weights, or the digestibility of feed did not change significantly.
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