Growth Performance and Shedding of Some Pathogenic Bacteria in Feedlot Cattle Treated with Different Growth-Promoting Agents
Eighty steers with a mean body weight of 319 kg were used in a study to evaluate the effect of a growth-promoting implant (trenbolone acetate plus estradiol benzoate), monensin, and oxytetracycline on the steer performance and shedding of some foodborne pathogens. The steers were allotted
to one of eight treatment combinations according to a randomized complete block design with 16 pens of five animals. Rectal fecal samples were collected before treatment commenced and over a period of more than 24 weeks to study the influence of treatments on the intestinal microbiology of
the animals. Results supported the beneficial effect of the hormonal implant on the performance of feedlot steers (average daily gain, feed efficiency, and fat thickness), on carcass characteristics (hot carcass weight, lean yield), and economic value of the carcasses (P < 0.01).
The levels of Escherichia coli in feces were not affected by treatments but remained high throughout the study period. Antibioticresistant isolates of E. coli were more frequently found as the study progressed but were not associated with any specific treatment. Also independently
of treatment, we observed a reduction over time in the shedding of Campylobacter and Yersinia during the feeding period, whereas the shedding of Enterococcus was increased. The results of this study confirmed the beneficial economic effect of growth-promoting agents in
beef production and showed that the agents tested did not specifically affect the overall microbial evolution of the animal gut. However, the study also showed, independently of the growth promoter used, the shedding of Campylobacter, Yersinia, and antibiotic-resistant E. coli
in the feedlot environment. These bacteria also may be found in the colonic tissue of steers at slaughter and might be a source of carcasses contamination.
Document Type: Research Article
Centre de Valorisation de la Diversité Microbienne (CEVDM), Département de Biologie, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1
Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, P.O. Box 90, 2000 Route 108 East, Lennoxville, Québec, Canada J1M 1Z3
Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1000, 6947 Highway 7, Agassiz, British Columbia, Canada V0M 1A0
Publication date: June 1, 2006
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