Simple SummaryAs of 1 December 2018, all medically important antimicrobials (MIA) for veterinary use in Canada required a prescription from a veterinarian for purchase. Cow–calf herds, while numerous and an important component of the food supply chain, have not been included in Canadian national on-farm antimicrobial-use surveillance. Studies of antimicrobial-use practices on extensively managed cow–calf operations are limited, and prior to this study, the effects of the regulatory changes on antimicrobials in cow–calf herds had not been reported. The objectives of this study were to describe antimicrobial use practices on cow–calf operations and any changes associated with the imposed regulations. Overall, antimicrobial use patterns in 2020 remain relatively unchanged in cow–calf herds after the regulatory changes and compared to previous comparable data from 2014. While almost all herds used antimicrobials, the frequency of antimicrobial use within cow–calf operations was low. Antimicrobials were primarily used for the treatment of respiratory disease in calves, neonatal diarrhea, and lameness in adults; however, most herds treated less than 5% of animals for these reasons. The most frequently reported antimicrobials, tetracyclines and florfenicol, were not classified as either high or very high importance to human health.AbstractDespite growing concern surrounding antimicrobial use (AMU) and the importance of cow–calf herds to the Canadian livestock industry, surveillance of AMU in cow–calf herds to inform antimicrobial stewardship programs has been sporadic. Producers from the Canadian Cow–Calf Surveillance Network (87%, 146/168) provided data and almost all reported AMU in at least one animal (99%, 145/146 herds) in 2019–2020. The most common reasons for AMU were treatment of respiratory disease in nursing calves in 78% of herds and neonatal diarrhea in 67% of herds, as well as for lameness in cows in 83% of herds. However, most herds treated <5% of animals for these reasons. Less than 2.5% of herds treated more than 30% of calves for either bovine respiratory disease or neonatal diarrhea and no herds treated more than 30% of cows for lameness. The most frequently reported antimicrobial was oxytetracycline in 81% of herds, followed by florfenicol in 73% of herds. Antimicrobials with very high importance to human health, such as ceftiofur, were used at least once by 20% of herds but were only used in >30% of nursing calves from one herd. Similarly, while 56% of herds used macrolides at least once, within-herd use was the highest in nursing calves where <4% of herds reported use in >30% of animals. Herds using artificial insemination and calving in the winter were more likely (p =0.05) to treat >5% of nursing calves for respiratory disease, suggesting the importance of vaccination programs for herds at risk. Overall, AMU was similar to previous Canadian studies; however, the percentage of herds using macrolides had increased from a comparable study in 2014.