Dairies and calf operations has been feeding calves non-salable milk from antibiotic-treated cows for decades.
But as concern for protecting the potency of antibiotics has grown, so too has the focus on the use of antibiotics in food-animal production and its potential crossover effects on human health. In short, the way we use antibiotics in raising our livestock is inter-related to the influence of all antibiotics, including those we use to treat our animals, as well as those that may be needed to treat our own families.
The heightened concern for the potential for developing antibiotic-resistant organisms has translated into changes at the farm level. Namely, they include Veterinary Feed Directive, which started in 2017, and the more recent veterinary oversight of other on-farm antibiotics.
And now, a new question: what about those traces of antibiotics in waste milk that are fed to young, developing calves? A group of researchers at the Federal University of Minas Gerais in Brazil conducted an in-depth study to explore whether feeding waste milk with antibiotics contributed to the development of antibiotic-resistant bacteria in calves. They also examined the effects of feeding various sources of whole milk on calves’ digestive development.
The study, published recently in the Journal of Dairy Science, included a total of 63 Holstein X Gyr crossbred dairy calves. They were assigned to one of three treatment groups of 21 calves each. After they all received colostrum and transition milk from their dams, the groups were reared on 6 liters per day of either (1) salable bulk tank milk (no antibiotics); (2) pasteurized waste milk; or (3) unpasteurized waste milk.
The researchers pointed out that while pasteurization does reduce bacterial loads in waste milk, it does not affect antibiotic residues. They also cited two previous studies in which “antibiotic residues were implicated in altered rumen microbiota and rumen fermentation profile due to variable quantities of waste milk reaching the rumen during feeding.”
A third of the calves (6 from each feeding group) were euthanized for tissue evaluation at 30 days of age, and the remaining animals (15 from each group) were euthanized at 60 days. The researchers examined fecal samples from 45 calves (15 per treatment) at 3, 30, and 60 days of age to evaluate the resistance patterns of 3 isolates of E. coli bacteria to 7 common on-farm antibiotics.
Those antibiotics included ampicillin, amoxicillin, ceftiofur, florfenicol, enrofloxacin, streptomycin, and tetracycline. Results of the evaluation included:
- Compared to the calves fed salable bulk tank milk, calves fed pasteurized and non-pasteurized waste milk showed a higher prevalence of resistant fecal E. coli at both 30 and 60 days of life.
- Non-pasteurized-waste-milk-fed calves showed resistance to 5 of the 7 antibiotics evaluated.
- Pasteurized-waste-milk-fed calves showed resistance to 4 of the 7 antimicrobials.
- Consistent among all three groups was the development of E. coli resistance to florfenicol, even though that antibiotic was not used to treat disease in the cow herd.
The researchers explained that the florfenicol resistance could be attributed to “resistance dissemination,” in which a gene that develops resistance to a specific drug develops alongside other resistant genes and then resistant bacteria are shared among animals.
Overall, the researchers underscored the need for vigilance in the way antibiotics are handled on-farm. “The presence of antimicrobial residues in waste milk raises significant concerns within the scientific community,” they said. “Waste milk’s use poses a risk by exerting selective pressure, fostering the emergence and spread of resistant bacteria in the intestinal microbiota of calves.”
They called for future research that tracks animals in transition from liquid to solid diets beyond weaning, noting, “understanding the effect of antibiotic residues is crucial for developing new feeding and residue-disposal strategies within calf-rearing systems.”
Your Next Read: Harnessing Technology to Elevate How We Care for Youngstock
