Bacillus stearothermophilus disk assay detection of penicillin in milk of dairy cows after postestrual intrauterine infusion.

Penicillin concentrations were quantitated in milk from cows after intrauterine infusions of various amounts of penicillin. Six healthy lactating Holsteins were assigned to 3 treatment groups in a complete randomized block design. Postestrual (12 to 48 hours) intrauterine infusions of potassium penicillin G in sterile diluent were given at 1 of 3 dosage levels: 2 X 10(6) IU (group I), 1.5 X 10(6) IU (group II), or 1 X 10(6) IU (group III). Milk samples were collected from 0 to 72 hours after infusion and were frozen until assayed for detectable antibiotic residues by the Bacillus stearothermophilus plate disk test. Intrauterine infusion of 1, 1.5, or 2 million IU caused detectable penicillin residues (greater than or equal to 5 mIU/ml) within 12 hours after infusion in milk from 15 of 18 cows. After 12 hours, no samples were positive in groups II or III and only 3 of 30 samples representing 2 of 6 cows were positive in group I. Using the B stearothermophilus plate disk test, a cow given 1 to 2 million IU of potassium penicillin G intrauterine will have detectable amounts of antibiotics in the milk within 12 hours. It is unlikely that herd composite milk samples would contain detectable concentrations of antibiotics unless a large percentage of the herd was infused or unless massive doses were administered to individual cows.