Rumen protected forms of methionine have been developed to facilitate rationing of dairy cows in term of digestible methionine. It is essential for rumen protected products that the proportion of the amino acid that resists ruminal degradation as well as the proportion available in the intestine is known. Methods based on the concentration of amino acid in blood have been developed to provide fast and effective assessment of protection / release coefficients. The objective of this trial was to test the efficiency with which four different sources of methionine could deliver methionine to the blood of dairy cows.

Two multiparous Friesian Holstein cows fitted with rumen and duodenal canulae and in their 10th week of lactation were used. They received a ration composed of corn silage ad libitum, soybean meal (2.5 kg/head/day) and concentrate (6.5 kg/head/day).