DEPLETION OF GENTAMICIN IN THE MILK OF HOLSTEIN COWS AFTER SINGLE ANDREPEATED INTRAMAMMARY AND PARENTERAL TREATMENTS

Twenty-four healthy Holstein cows, 2.72 +/- 0.64 (mean +/- SD) years o ld, weighing 603.96 +/- 73.22 kg (mean +/- SD), and representing vario us levels of milk production, were used to determine the depletion of gentamicin (GT) in milk. The cows had not received antibiotics or othe r drugs that could interfere with study for at least 60 days before th e beginning of the investigation. The cows were divided into six group s (n = 4) and treated with single (treatments A, B and C) or repeated (treatments D, E and F) doses of GT. Cows were acclimated for 7 days b efore administration of GT and milked twice a day at 12-h intervals (0 6.00 hours, 18.00 hours) throughout the duration of the study. Control milk samples were obtained after the arrival of the cows and assayed to establish their GT free status. On day 1 of each treatment, a basel ine milk sample was collected from the milk produced (06.00 hours) by each cow. A single dose of GT was administered intramammarily (A, i.m. m. left front quarter, 500 mg), intravenously (B, i.v., 5 mg/kg body w eight) or intramuscularly (C, i.m., 5 mg/kg body weight). Cows in trea tments D (i.m.m., 500 mg), E (i.v., 5 mg/kg body weight) and F (simult aneous i.m.m. 500 mg plus i.v. 5 mg/kg body weight) were treated twice a day for 5 consecutive days just after the morning and evening milki ngs. Milk samples from individual cows were cllected every day after e ach milking during and after dosing until GT concentration in the milk was below the safe level of less than or equal to 30 ng/mL. The conce ntration of GT in milk was determined by a high-performance liquid chr omatographic procedure. Depletion of GT to a concentration less than o r equal to 30 ng/mL occurred at the seventh (84 h), third (36 h), thir d (36 h), eleventh (132 h), third (36 h) and nineteenth (228 h) post-d osing milking, for cows in treatments A, B, C, D, E and F respectively . The highest mean +/- SEM) concentrations of GT were 14 710 +/- 1213. 89, 167.87 +/- 46.94 and 91.62 +/- 14.55 ng/mL, measured in the first milking post dosing (12 h) for cows in treatment A, B and C respective ly; for cows in treatments D, E and F, during the dosing period, they were 14067.50 +/- 2989.09, 446.07 +/- 100.99, and 22900 +/- 2843.66 ng /mL and occurred at the seventh, third and eighth milking respectively , Because GT is not approved for use in dairy cattle and because of th e long depletion time associated with some possible treatments, illega l and extra-label use is likely to cause residues in milk.