Feeding waste milk is a common practice on many U.S. dairy farms and calf facilities. USDA estimates more than half of all dairies feed whole milk, representing more than 70% of the heifer calves raised in the U.S.
“Waste” milk might contain antibiotics from treated cows that have not met their drug withdrawal period. Jim Quigley, technical and research manager for Provimi North America, Brookville, Ohio, says a question of antibiotic resistance developed from feeding waste milk is often raised. “As knowledge and awareness of antibiotic resistance has grown, so too have questions about the practices that might be contributing to it in food-animal production,” Quigley says.
In an issue of Calf Notes, Quigley summarized a study published in the Journal of Dairy Science. Researchers at the University of Barcelona evaluated preweaned calves on eight farms—four fed waste milk and four fed milk replacer.
Fecal and nasal swabs were collected at about 42 days of age from about 20 calves per farm. They analyzed the samples for the presence of antibioticresistant bacteria. Among the findings:
- Calves receiving waste milk had an increased presence of fecal E. coli resistant to enrofloxacin, florfenicol, streptomycin, doxycycline and erythromycin. Feeding waste milk also increased the percentage of multidrug resistant fecal E. coli.
- Resistance to a specific antibiotic did not necessarily correlate to use of that antibiotic on the same farm. The authors suggest “the use of a particular antimicrobial could select for resistance to other antimicrobials within a bacterial population.” Another theory is resistant organisms might have been transmitted via contact with animals from another farm or by calves consuming contaminated feed or water.
- There was no significant resistance to a family of antibiotics that is used frequently on most dairy farms—the beta-lactams, including amoxicillin and ceftiofur. This lack of resistance was consistent, regardless of whether or not calves were fed waste milk.
- Nasal swabs were evaluated for the presence of resistant Pasteurella multocida. Only about 36.5% of samples contained P. multocida, and resistance among those samples was minimal. Only resistance to colistin was detected in calves fed waste milk, although only one farm used colistin to treat sick calves.
“Different types of bacterial mutations and horizontal (bacteria-to-bacteria) transmission of resistance genes have been reported and were hypothesized as a reason for the colistin resistance in P. multocida in this study,” Quigley says.
“Feeding waste milk appears to increase the resistance to some antibiotics in fecal bacteria of calves,” he continued. “This study also suggests that horizontal transmission of genetic resistance from other bacteria in the environment or other animals may contribute to antibiotic resistance in young calves.”