Degradation of methionine hydroxy analog in the rumen of lactating cows.

Four lactating cows fitted with T-type cannulae in the proximal duodenum were utilized in a 4 X 4 Latin square design to study rumen microbial degradation of methionine hydroxy analog, a methionine supplement. A diet consisting of 55% concentrate and 45% corn silage was fed ad libitum four times daily. The four treatments were 1) control, no methionine hydroxy analog, 2) methionine hydroxy analog in the form of a calcium salt, 3) methionine hydroxy analog in the acid form, and 4) DL-methionine. The amino acids were incorporated into a grain mix, which was top-dressed. All diets were isonitrogenous. Twelve samples of duodenal digesta and fecal matter were collected during the last 3 d of each of the four 14-d periods. Samples were composited for analysis. Microbes either altered or degraded 99% of the methionine hydroxy analog in the rumen, since recovery of the analog in duodenal digesta was less than 1% of the amount fed for both the acid form and the calcium salt.