Frequency of Antimicrobial-Resistant Fecal Escherichia coli Among Small, Medium, and Large Beef Cow–Calf Operations in Florida

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Antimicrobial resistance (AMR) is a growing threat to both animal and human health. This study investigated the occurrence of AMR in Escherichia coli isolated from fecal samples collected from beef cow–calf operations and compared the frequency of antimicrobial-resistant fecal E. coli among small, medium, and large beef cow–calf operations in Florida, United States. The study included nine beef cow–calf operations. Between December 2023 and April 2024, a total of 743 fecal samples were collected from cows (n = 429) and calves (n = 314), either directly from the rectum or from fresh voided feces. From these samples, 3475 E. coli isolates (five isolates/animal) were subjected to antibiotic susceptibility testing using the Kirby–Bauer disk diffusion method. A panel of eight antibiotics was used to assess AMR profiles. Irrespective of farm size, cows and calves showed higher resistance to streptomycin (47% or 330/695), oxytetracycline (46% or 319/695), sulfadimethoxine (42% or 291/695), ampicillin (41% or 283/695), and florfenicol (18% or 126/695). In contrast, lower resistance frequencies were observed for gentamicin (4% or 27/695), ceftiofur (3% or 19/695), and trimethoprim–sulfamethoxazole (3% or 18/695). Animals from medium (OR = 1.67) and large operations (OR = 2.70) had greater odds of harboring E. coli resistant to ≥1 antibiotic than those from small operations. Streptomycin (medium OR = 1.92; large OR = 4.17) and sulfadimethoxine resistance (medium OR = 1.64; large OR = 3.45) were also more frequent in medium and large operations, respectively. Additionally, E. coli resistance to florfenicol was higher in calves (OR = 3.57) than in cows, after controlling for farm size. This study provides new insights into AMR patterns in fecal bacteria from beef cow–calf operations in Florida and can help producers and veterinarians develop informed strategies for monitoring and mitigating AMR.