Transfer of sulphamethazine from contaminated dairy feed to cows' milk

Four groups of four healthy mid-lactation Friesian cows were fed a compound feedingstuff containing either 2, 10 or 250 mg sulphamethazine/kg, corresp onding to 0 2, 10 and 250 per cent of the therapeutic inclusion rate in rat ions for pigs, at a flat rate of 3 kg twice daily far 21 days, followed by a seven-day withdrawal period. The cows were machine-milked twice daily and pooled milk samples from each cow were analysed by a commercially availabl e microbiological assay with a sensitivity of 100 pg/litre and by a high pe rformance liquid chromatography (HPLC) procedure with a limit of detection of 10 pg/litre. Na sulphamethazine was detected by HPLC in the milk samples taken from any of the cows fed the concentrate containing 2 or 10 mg/kg. T he milk samples from ail four cows fed the highest concentration of sulpham ethazine contained from 21 to 120 mu g/litre while they were being fed the contaminated concentrate, The cow with the highest concentrations of sulpha methazine was the only one which repeatedly tested positive by the microbio logical assay. The concentration of sulphamethazine declined rapidly during the withdrawal period and the drug was not detectable by either method in samples taken from two days after the contaminated feed was withdrawn.