Effect of supplementing corn- or barley-based feedlot diets with canola oil on faecal shedding of Escherichia coli O157:H7 by steers S.J. Bach 1 , L.J. Selinger 2 , K. Stanford 3 and T.A. McAllister 2 1 Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Summerland, BC, 2 Lethbridge Research Centre, Agriculture and Agri-Food Canada, Lethbridge, AB, and 3 Alberta Agriculture, Food and Rural Development, Lethbridge, AB, Canada 2004/0894: received 30 July 2004, revised 8 September 2004 and accepted 14 September 2004 ABSTRACT S.J. BACH, L.J. SELINGER, K. STANFORD AND T.A. MCALLISTER. 2004. Aims: This study was conducted to evaluate the effect of supplementing barley- or corn-based diets with canola oil on faecal shedding of Escherichia coli O157:H7 by experimentally inoculated feedlot cattle. Methods and Results: Four groups of yearling steers fed on barley- or corn-based feedlot diets containing 0% (BA; CO) or 6% canola oil (BA-O; CO-O) were inoculated with 10 10 CFU of a mixture of four nalidixic acid-resistant strains of E. coli O157:H7. The inoculated strains were tracked in oral (mouth swab) and environmental (water, water bowl interface, feed, faecal pat) samples by enrichment and immunomagnetic separation (IMS) for 12 weeks, and in rectally collected faecal samples for 23 weeks (enumeration by dilution plating for 12 weeks; detection by IMS for a further 11 weeks). Levels of E. coli O157:H7 shed in faecal samples over the course of the enumeration period were similar (P ¼ 0Æ14) among treatments. Disappearance of the inoculated strains from faeces was more rapid (P ¼ 0Æ009) with barley than with corn, but shedding levels at the end of the enumeration period were similar (P ¼ 0Æ21) across grain types. Canola oil supplementation did not affect (P ¼ 0Æ71) the rate of disappearance of E. coli O157:H7 from faeces. The numbers of steers culture positive for E. coli O157:H7 during the enumeration period were similar (P ¼ 0Æ57) among treatments. During the 11-week detection period, however, more (P <0Æ001) steers were E. coli O157:H7-positive in the BA group (15/64) than in BA-O (two of 64), CO (two of 56), or CO-O (one of 56). The organism was present in two of 48 water samples (both CO-O), one of 48 water bowl swabs (BA-O), four of 48 feed samples (two of 12 BA; two of 12 CO-O), 30 of 48 pen floor faecal pat samples, and 296 of 540 mouth swabs (81/144 BA, 80/144 BA-O, 74/126 CO and 61/126 CO-O). Conclusion: Supplementing corn or barley-based diets with canola oil did not affect shedding of E. coli O157:H7 by feedlot cattle. Significance and Impact of the Study: High-shedding individuals (i.e. Ôsuper sheddersÕ) may be responsible for disseminating E. coli O157:H7 among penmates. Faeces on pen floors appears to be a more significant source of infection than are feed or drinking water. Keywords: barley, canola oil, cattle, corn, diet, Escherichia coli O157:H7. Correspondence to: Tim McAllister, Agriculture and Agri-Food Canada Research Centre, PO Box 3000, Lethbridge, AB, Canada T1J 4B1 (e-mail: mcallister@agr.gc.ca). ª 2004 Minister of Public Works and Government Services Canada Journal of Applied Microbiology 2005, 98, 464–475 doi:10.1111/j.1365-2672.2004.02465.x