24.02.2021: The use of antimicrobial drugs and the subsequent selection for antibiotic-resistant bacteria is a global health problem affecting humans, animals and the environment. A research team from Vetmeduni has examined a possible factor influencing this problem in more detail, namely the feeding of waste milk – milk that does not meet the legal requirements for human consumption – to calves. The researchers argue that more attention should be paid to the possible negative consequences of this feeding practice and that alternative strategies must be explored.

When dairy cows are treated with medicinal products, a legally-prescribed waiting period applies during which the milk used for human consumption. The milk produced by the cow during this time is sometimes disposed of, but is more commonly fed to calves. As a source of high-quality nutrients, milk is an excellent feedstuff for calves. However, the antimicrobial residues contained in waste milk also make it a potential cause for concern, as this practice may contribute to the development and spread of antimicrobial resistance. In January 2017, the European Food Safety Authority (EFSA) published a Scientific Opinion on the risk of antibiotic-resistant bacteria developing in dairy calves. EFSA experts estimate that around 1% of the milk produced in the European Union is classified as waste milk that is unfit for sale and is probably fed to calves.

Greater consideration needed of negative effects of feeding waste milk to calves

In a recent review for the journal Pathogens, a team led by Clair Firth from the Institute of Food Safety, Food Technology and Veterinary Public Health at Vetmeduni analysed the relevant scientific literature on the feeding of waste milk to calves. The scientific overview is based on a total of 19 research papers, published in international journals between 2016 and 2020, as well as a summary of the EFSA Scientific Opinion. The vast majority of the studies assessed the faecal bacteria E. coli isolated from dairy calves, in particular the effect of waste milk feeding on the prevalence of its antimicrobial resistant forms. The research findings include some positive effects on daily liveweight gain and other advantages for calf health from feeding waste milk compared to milk replacer. Nevertheless: “The feeding of waste milk is accompanied by negative effects, such as the selection for antimicrobial-resistant bacteria, a shift in the intestinal microbiome and possible negative consequences on global public health. We therefore recommend that these factors should always be taken into consideration when feeding waste milk to calves,” says Firth. The experts also consider it essential to develop alternative strategies for the safe use of waste milk in the future, so that the high-quality nutrients they contain do not go to waste.

Complex phenomenon requires further analysis

The studies included in the review presented a variety of divergent outcomes. While feeding milk containing antimicrobial residues appears to increase the excretion of antimicrobial-resistant bacteria in dairy calves, such shedding is frequently short-lived. Although changes in the calves’ microbiome were often reported following waste milk feeding, it is not possible at present, based on the available research, to draw a clear conclusion as to how this affects the calves’ continued health. In addition, the possible transmission of antimicrobial-resistant bacteria from waste milk to calves appears to be a contributing factor to this highly complex problem. There is no question that sick cows must be treated adequately, with antimicrobials when necessary. The production of waste milk is unavoidable in this process. “Further studies are needed to enable us to recommend a course of action regarding the treatment of waste milk. This will allow us to gain a more comprehensive picture of possible correlations and to better assess which risks are actually associated with feeding waste milk to calves,” says Firth. In addition, research should be conducted on how to treat waste milk so that the valuable nutrients can be utilised while avoiding possible risks to health.

The article “The Effects of Feeding Waste Milk Containing Antimicrobial Residues on Dairy Calf Health” by Clair L. Firth, Katrin Kremer, Thomas Werner and Annemarie Käsbohrer was published in Pathogens.

Note: The press release forms part of the priority communication related to the UN Sustainable Development Goal “Zero Hunger”.