Effects of feeding a simulated waste milk on growth, health, fecal microbiota, and antibiotic resistance in dairy heifer calves.

Feeding waste milk, a common practice in dairy farming, exposes calves to subtherapeutic levels of antimicrobials, potentially contributing to antibiotic resistance-a growing concern globally. Many dairy farmers, including those in Ireland, continue this practice, feeding waste milk from antibiotic-treated cows to calves. Although previous studies have linked waste milk feeding to changes in calf growth and health during the preweaning period, its effects postweaning remain unclear. This study examined how the duration of antimicrobial exposure at levels equivalent to those found in waste milk influences health and growth outcomes of dairy heifer calves both before and after weaning. It also assessed the prevalence of extended-spectrum β-lactamase (ESBL)-producing antimicrobial-resistant Escherichia coli in feces and changes in the fecal microbiota over time. To mimic waste milk, as derived from a cow treated with an intramammary suspension of antibiotics, a simulated waste milk (SWM) was prepared by adding amoxicillin (1.68 mg/L) and neomycin (2.28 mg/L) to a conventional milk replacer (MR). The study employed a randomized block design with 87 dairy heifer calves assigned to 1 of 3 treatments: (1) long-term antibiotic (LTA), with calves fed SWM until weaning at 12 wk; (2) short-term antibiotic (STA), with SWM fed from 3 to 5 wk; and (3) control (CONT), with calves fed antibiotic-free MR. Calves were weighed weekly, and health scores, including fecal scores (tail and hindquarters cleanliness as diarrhea indicator), were recorded twice per week. Fecal and blood samples were collected to analyze microbiome changes and the shedding of antimicrobial resistance. Blood samples were taken to measure systemic inflammation, using serum amyloid A as a biomarker. Results indicated that SWM feeding did not affect average daily gains before or after weaning. However, higher fecal scores were observed in the LTA group during weaning and after weaning in the STA group. Antibiotic-resistant isolates were present in all groups, with the highest prevalence in LTA. Fecal microbiota analysis revealed treatment-specific microbial community variations, with an increase of Enterococcus faecium genes resistant to macrolide, aminoglycoside, and tetracycline antibiotics in LTA and STA compared with CONT. In summary, SWM feeding did not significantly affect growth or overall health, but it was associated with increased fecal shedding of resistant bacteria and some changes in the microbiota, indicating potential long-term implications for antimicrobial resistance in dairy herds.