Residues in colostrum following antibiotic dry cow therapy.

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.