Supplementary Materials: The 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 sub-therapeutic 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. While previous studies have linked waste milk feeding to changes in calf growth and health during the pre-weaning period, its effects post-weaning 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 pre- and post-weaning. It also assessed the prevalence of extended-spectrum β-lactamase (ESBL) 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 one of three treatments: (i) Long-term antibiotic (LTA), with calves fed SWM until weaning at 12 weeks; (ii) Short-term antibiotic (STA), with SWM fed from three to five weeks; and (iii) 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, and. 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 pre- or post-weaning. However, higher fecal scores were observed in the LTA group during weaning and post-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 increased resistance genes in the bacteria Enterococcus faecium to macrolide, aminoglycoside, and tetracycline antibiotics in LTA and STA compared to CONT. In summary, SWM feeding did not significantly affect growth or overall health, 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.
