Effects on Escherichia coli O157:H7 Prevalence and Levels from Feeding Two Formulations of Direct-Fed Microbial to Feedlot Cattle

Preharvest interventions are critical to reduce beef carcass contamination with Escherichia coli O157:H7 during processing. We determined the effects of feeding two direct-fed microbials (DFMs), Bovamine Defend and 10-G, to beef cattle to suppress the prevalence and load of E. coli O157:H7 in the gastrointestinal tract. Each DFM was fed daily to eight pens of cattle (100 animals/pen) from May to September. Fecal swabs were collected from 23 to 25 animals/pen at midpoint (∼70 days on feed) and at the end of the feeding period (140 days on feed). Hide swabs were collected at the final sampling. The prevalence of E. coli O157:H7 in feces ranged from 8% ( n = 193) in the samples collected at midpoint from the Bovamine Defend group to 25% ( n = 192) in the samples collected at the end of the feeding period from the 10-G group. After controlling for the effect of sampling occasion, cattle that were fed 10-G had 70% higher odds of E. coli O157:H7 positivity than cattle that were fed Bovamine Defend (odds ratio = 1.7; 95% confidence interval = 1.1–2.7; p = 0.019). Overall, 15 of 386 (4%) and 27 of 384 (7%) fecal samples had quantifiable numbers of E. coli O157:H7 from the Bovamine Defend and 10-G groups, respectively. The total number of super-shedders from both sampling occasions did not statistically differ ( p > 0.50) between groups. The prevalence of E. coli O157:H7 on hides (Bovamine Defend, 67.7% [ n = 192]; 10-G, 60.4% [ n = 192]) did not statistically differ ( p = 0.56) between treatment groups. Neither treatment prevented animals from super-shedding, much less shedding >200 colony forming units of E. coli O157:H7/swab, a shedding level previously associated with high prevalence of cattle hide contamination, suggesting that neither treatment may be sufficiently efficacious as a preharvest intervention.