Differences in the fecal microbiota of dairy calves reared with differing sources of milk and levels of maternal contact.

The practice of rearing cows and calves together is gaining popularity on dairy farms, with different systems currently under assessment in mainland Europe, the United Kingdom, and Oceania. Research into the effects of cow-calf rearing has primarily focused on direct health and welfare implications, and little work has examined the role of different rearing paradigms on calf microbiota. We trialed a cow-calf rearing system on a Canadian dairy farm and compared fecal microbiota of these calves with the microbiota of calves reared according to the conventional practice of the same farm (separated from the dam and fed waste milk). At 4 wk, the conventionally reared calves had reduced relative abundance of and higher relative abundance of other taxa, including , , and We also detected predicted functional differences, such as reduced l-tryptophan biosynthesis in conventionally reared calves. These results suggest that maternal contact may influence the calf microbiota, but the observed differences are also likely related to other aspects of the rearing environment independent of maternal contact (e.g., potential exposure to antibiotic residues in waste milk). These findings provide preliminary evidence of the effects of early rearing environments on the establishment of the dairy calf fecal microbiota. This research is needed, given the critical role of the bovine gut microbiome in behavioral, metabolic, and immune development.