To examine the effect of
ionophore supplementation on fecal shedding of Escherichia coli O157:H7 and Salmonella, crossbred beef calves (n=113; mean
body weight [BW], 243 kg) were fed a
mineral supplement with
ionophore (1.76 g
lasalocid/kg) for 61 days (d). Control calves received an identical
mineral supplement without
lasalocid. Calves were pastured on fescue/bermudagrass paddocks and supplemented with a corn/wheat midds/soybean meal supplement (1.5% of BW/d). Upon arrival, cattle were fed a commercial receiving ration containing 1 g
chlorotetracycline/kg for 10 d. Sick calves were administered one or a combination of the following: Nuflor (
florfenicol),
Baytril (eurofloxacin), Micotil (
tilmicosin), or LA 200 (
oxytetracycline). Fecal samples were collected immediately prior to
ionophore supplementation, approximately midway and at the end of the experimental period (60 d total
ionophore feeding) for isolation of E. coli O157:H7 and Salmonella. Putative fecal coliforms were also isolated at these sampling times and examined for antimicrobial susceptibility. The study was replicated over a two year period (year 1, n=53 head; year 2, n=60 head).
Ionophore supplementation had no effect (p>0.10) on the incidence of calves shedding E. coli O157:H7 or Salmonella. The percentage of calves shedding E. coli O157:H7 varied throughout the experimental period from 0 to 30%, while Salmonella was cultured from only three calves over the 2-year experimental period. Antimicrobial susceptibility profiles of putative fecal coliforms were consistent with
antibiotic treatments administered during the study (observed resistance to
chlortetracycline,
florfenicol,
oxytetracycline), while only one treatment effect was observed.
Ionophore treatment resulted in a significantly higher number of coliform isolates resistant to
ampicillin compared to controls in year 1, but not year 2. A number of fecal coliform isolates demonstrated resistance to multiple
antibiotics, however, this was not affected (p>0.10) by
ionophore supplementation.
Mineral intakes, BW gain, and the number of sick calves were similar (p>0.10) among treatments.
Ionophore supplementation had no affect on fecal shedding of E. coli O157:H7 or Salmonella and a negligible impact on antimicrobial susceptibility patterns of fecal coliforms in beef calves.