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Transfer of chiortetracycline from contaminated feedingstuff to cows' milk
  1. J. D. G. McEvoy, MVB,PhD, MRCVS1,
  2. H. C. Higgins, BA1,
  3. D. G. Kennedy, BSc, PhD1 and
  4. C. S. Mayne, BAgr, PhD2
  1. 1 Residues Department, Department of Agriculture for Northern Ireland, Veterinary Sciences Division, Stoney Road, Stormont, Belfast BT4 3SD
  2. 2 Agriculture Research Institute of Northern Ireland, Large Park, Hillsborough, Co Down BT26 6DR

Abstract

Three groups of four Friesian cows in mid-lactation were fed a compound feedingstuff contaminated with 2, 10 or 300 mg chlortetracycline/kg for 21 days, 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 pooled milk samples from each cow were analysed by high performance liquid chromatography (HPLC) with a detection limit of 50 μg chlortetracycline/litre. Chlortetracycline was deteded in only two milk samples taken from one of the animals fed feed containing 300 mg chlortetracycline/kg; and both contained less than the maximum residue limit (MRL) specified by the European Union (100 μg/litre). All the milk samples were also analysed by the Delvotest sp microbiological assay, which has a detection limit of 300 μg chlortetracycline/litre. During the treatment period, this method gave four presumptive false-positive results, because they were not confirmed by HPLC. Seleded daily pooled samples from each treatment group were also analysed by the semi-quantitative Charm II radioreceptor assay with a detection limit of 10 μg chlortetracycline/litre. Immunoreactive chlortetracycline was detected 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. However, there was a trend towards a higher milk fat concentration (P<0.09) and a lower milk protein concentration (P<0.07) with increasing concentration of chlortetracycline in the diet.

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