Effect on the gut microbiome of feeding waste milk from cows to calves. Waste milk and gut microbiome in dairy calves

Waste milk (WM) is a by-product of the dairy industry that cannot be sold for human consumption, and a convenient alternative for the farmer is using it as calf food. Still, its nutritionally unbalanced composition and the presence of pathogens, toxins, and antimicrobial agents can lead to intestinal problems with possible lasting effects. We investigated the short-term and medium-term consequences of feeding a pooled, standardized WM from cows with chronic bacterial mastitis on intestinal health, weight gain, and hindgut microbiota in an 8-week feeding trial. After 3 days of colostrum, 6 calves received WM, and 6 received normal milk (NM) for 2 weeks. Then, all calves received a weaning diet of milk whey and starter feed for further 6 weeks. WM calves had a significantly higher incidence of diarrhea than NM calves in the first two weeks of the trial (odds ratio 6.54) and significantly lower body weight until the end of the trial. The 16S rDNA marker gene analysis revealed a sharp decrease in alpha-diversity of the hindgut microbiota in WM calves. Alpha-diversity continued to decrease significantly even after two weeks of weaning diet and then recovered slowly with time, but minor differences were still present at trial completion. Concerning beta-diversity, several taxonomic groups were significantly different in NM versus WM at all time points, from the genus level to the phylum level. Among others, Negativicutes, Lachnospiraceae, Ruminococcaceae, Allisonella, Fusicatenibacter, Prevotella, Acetatifactor, and Shuttleworthia were higher in NM during the trial, while Alphaproteobacteria, Deltaproteobacteria, Catenibacterium, Veillonella, and Rhodococcus were higher in WM. Remarkably, after six weeks under the weaning diet, the phylum Saccharibacteria was still significantly higher in NM. In conclusion, feeding pre-weaned calves with WM from cows with chronic mastitis compromised intestinal health, weight gain, and hindgut microbiota diversity. Significant taxonomic changes up to the phylum level persisted in the gut microbiota even after 6 weeks of weaning diet, suggesting that this practice may lead to prolonged consequences on the calf intestinal microbiota, possibly interfering with the calf health and nutrient utilization efficiency in the longer term.