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Cows from five dairy herds were used to determine persistence of antibiotic residues in colostrum and milk following dry cow therapy. Cows were treated in all quarters at drying off with antibiotics approved for use for nonlactating cows. Antibiotics procaine penicillin G plus dihydrostreptomycin, novobiocin, cloxacillin, or cephapirin were compared with no treatment. Composite colostrum samples were collected from each cow at first milking after parturition. Samples were screened for residues by Delvotest P. Colostrum samples positive by Delvotest also were tested by Bacillus stearothermophilus disc assay. Four of 186 colostrum samples from cows treated with antibiotics at drying off were positive for residues by Delvotest. Only one was confirmed positive by disc assay following heat treatment. All colostrum samples from 48 cows not treated were negative. Samples of first marketable milk also were collected. Over 96% of milk samples from cows treated at drying off and 100% of milk samples from cows not treated were negative for residues by Delvotest. If manufacturer's recommendations are followed, antibiotic residues in colostrum and milk following dry cow therapy with products in our study should not be a significant problem.

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.