Thirty-six two-week-old healthy Holstein-Friesian calves weighing between 52 and 58 kg were divided at random into three groups of 12; group A calves were given a single oral bolus containing 2˙5 g sulphathiazole and 1 g trimethoprim in a sustained-release formulation; group B received the same doses of the drugs but the trimethoprim was not in a sustained-release formulation; group C received a bolus containing 2˙5 g sulphathiazole and 0˙5 g conventional trimethoprim. Blood samples were collected at intervals for two days, the serum was separated and the composite antibacterial activity profiles of the mixture were analysed by an agar-diffusion microbiological method. The mean maximum activities in the serum of the three groups were 23˙4 μg/ml in group A, 9˙25 μg/ml in group B and 8˙01 μg/ml in group C. The mean areas under the curves of the serum activity time curves were 838 μg/ml/hour in group A, 216 μg/ml/hour in group B and 182 μg/ml/hour in group C.
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