Stanford, K, Gibb, D. and McAllister, T. A. 2013. Evaluation of a shelf-stable direct-fed microbial for control ofEscherichia coliO157 in commercial feedlot cattle. Can. J. Anim. Sci. 93: 535-542. A direct-fed microbial (DFM) registered for use in cattle in Canada containing Lactobacillus acidophilus strain BT-1386 and a Saccharomyces cerevisiae yeast autolysate was evaluated for control of E. coli O157. Weaned calves entered the feedlot in October and November and in January were sorted into Control (12 pens with a total of 2170 calves) and DFM treatment groups (10 pens with a total of 2040 calves). Although targeted dosage of L. acidophilus was 9 log₁₀ colony forming units (CFU) head^-1 d^-1, analyses after storage at ambient temperature showed an average dose of 8.6 Log₁₀ CFU head^-1 d^-1 and demonstrated stability of DFM over the range of temperatures encountered (-32.6 to 32.9°C) during storage. Calves entering the feedlot had low prevalence (0.8%) of E. coli O157 in feces, which increased to 11.2% in January. A 47°C range in ambient temperature for that month may have stressed cattle and led to increased shedding of E. coli O157 compared with seasonal norms. Comparing hide swabs collected at initiation of DFM feeding with those at shipping for slaughter, prevalence of E. coli O157 declined (P<0.05) in cattle fed DFM, although prevalence of E. coli O157 in hide swabs from Control and DFM-treated cattle did not differ at any time. As well, numbers of E. coli O157 and prevalence of the organism in fecal pats did not differ among treatments. Colonization of calves with E. coli O157 prior to DFM feeding likely reduced efficacy of DFM in the present study. Additional information regarding timing of feeding DFM relative to interactions among organisms within the gastrointestinal tract of cattle are required to ensure consistent efficacy of DFM for pre-harvest control of E. coli O157.