A mail survey was conducted of dairy producers who had received a positive bulk milk antibiotic residue test result in a two-year period (1987–88) of government monitoring (case farms) and farms that were negative for all tests conducted in the same period (control farms). Farmers were asked to complete questionnaires designed to determine dairy management practices, as well as, antibiotic handling and residue prevention methods. Using multiple logistic regression analysis, and adjusting for the size of the milking herd, the following factors were associated with increased risk of antibiotic residues in milk: the use of part-time assistance in milking, use of a milking parlor and increased estimated frequency of intramammary antibiotic treatments. Unconditionally, significantly more control farmers used separate equipment to milk treated cows rather than simply attempting to divert milk from the bulk tank. Controls were also more likely to vary the withholding time of milk for different drugs. Other significant differences between cases and controls with respect to residue prevention methods were observed, however, some of these may have been due to changes instituted on case farms after the antibiotic residue violations occurred. For example, significantly more case than control farmers reported using on-farm residue test kits and marking of treated animals as residue prevention methods and more case farmers believed that failure to keep good records of treatment was an important factor in residue occurrence. No significant differences were observed in the proportions of case and control farms that used medicated feed, in the number of people employed on the farm, or in the general knowledge of antibiotic residue prevention.

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