Transfer of chlortetracycline from contaminated feedingstuff to cows' milk

Three groups of four Friesian cows in mid-lactation were fed a compound fee ding stuff contaminated with 2, 10 or 300 mg chlortetracycline/kg for 21 da ys, and were then fed an uncontaminated diet for seven days. A fourth group of four cows was fed an uncontaminated diet throughout the study. Daily po oled milk samples from each cow were analysed by high performance liquid ch romatography (HPLC) with a detection limit of 50 mu g chlortetracycline/lit re. Chlortetracycline was detected in only two milk samples taken from one of the animals fed feed containing 300 mg chlortetracycline/kg, and both co ntained less than the maximum residue limit (MRL) specified by the European Union (100 mu g/litre). All the milk samples were also analysed by the Del votest sp microbiological assay, which has a detection limit of 300 pg chlo rtetracycline/litre. During the treatment period, this method gave four pre sumptive false-positive results, because they were not confirmed by HPLC. S elected daily pooled samples from each treatment group were also analysed b y the semi-quantitative Charm II radioreceptor assay with a detection limit of 10 mu g chlortetracycline/litre. Immunoreactive chlortetracycline was d etected only in the animals fed feed containing 300 mg chlortetracycline/kg and several of the results exceeded the Eu MRL during the treatment period . No significant treatment effects on animal performance were observed. How ever, there was a trend towards a higher milk fat concentration (P<0.09) an d a lower milk protein concentration (P<0.07) with increasing concentration of chlortetracycline in the diet.