Inclusion of distillers grains (DG) in cattle diets has been shown to increase fecal shedding of Escherichia coli O157:H7. It is hypothesized that altered gut fermentation by DG may be responsible for the positive association. Therefore, feed additives affecting ruminal or hindgut fermentation of DG also may affect fecal shedding of E. coli O157:H7. The objectives of the study were to evaluate effects of monensin (33 or 44 mg/kg of DM), supplemental urea (0, 0.35, or 0.70% of DM), and ractopamine (0 or 200 mg/steer daily administered during the last 42 d of finishing) in a steam-flaked corn grain-based diet containing 30% wet sorghum DG on fecal shedding of E. coli O157:H7. Seven hundred twenty crossbred beef steers, housed in 48 pens (15 steers/pen), were assigned to dietary treatments in a randomized complete block design with a 2 × 3 × 2 factorial treatment arrangement. Fresh pen floor fecal samples (10 per/pen) were collected every 2 wk for 14 wk (July through November) and cultured for E. coli O157:H7. Isolation of E. coli O157:H7 was by selective enrichment of fecal samples in an enrichment broth, immunomagnetic separation, followed by plating onto a selective medium. Samples that yielded sorbitol-negative colonies, which were positive for indole production, O157 antigen agglutination, and contained rfbE, fliC, and stx2 were considered positive for E. coli O157:H7. Fecal prevalence data were analyzed as repeated measures using negative binomial regression to examine effects and interactions of sampling day, urea, monensin, and ractopamine. Mean fecal prevalence of E. coli O157:H7 was 7.6% and ranged from 1.6 to 23.6%. Cattle fed monensin at 44 mg/kg of feed had less (P = 0.05) fecal E. coli O157:H7 prevalence than cattle fed 33 mg/kg (4.3 vs. 6.8%). Although the reason for the reduction is not known, it is likely because of changes in the microbial ecosystem induced by the greater amount of monensin in the hindgut. Supplemental urea at 0.35 or 0.70% had no effect (P = 0.87) on fecal shedding of E. coli O157:H7. Fecal prevalence of E. coli O157:H7 were 5.3, 5.7, and 5.9% for groups fed 0, 0.35, and 0.7% urea, respectively. The inclusion of ractopamine at 0 or 200 mg/(animal•d) had no effect (P = 0.89) on fecal prevalence of E. coli O157:H7 (4.4 vs. 4.0%). Additional research is needed to confirm the reduction in fecal shedding of E. coli O157:H7 in cattle fed monensin at 44 mg/kg of feed compared with cattle fed 33 mg/kg of feed.

J. Anim. Sci. Vol. 88, E-Suppl. 2/J. Dairy Sci. Vol. 93, E-Suppl. 1/Poult. Sci. Vol. 89, E-Suppl. 1 1085 C-di-GMP signaling pathways are critical for acid resistance of E. coli O157:H7. M. J. Zhu*1, B. L. Wang1, W. Yue1, V. K. Koseoglu1, H. Wang1, X. Fang2, W. J. Means1, R. J. McCormick1, and M. Gomelsky2, 1Department of Animal Science, Laramie, WY, 2Department of Molecular Biology, University of Wyoming, Laramie.

Four experiments were conducted to investigate the effects of ractopamine hydrochloride (RAC) on ruminal fermentation and proteolysis. In Exp. 1, in vitro gas and VFA production was measured in flasks incubated with 0, 0.226, 2.26, 22.6, and 226.0 mg of RAC/L of buffered ruminal fluid. Ractopamine hydrochloride had a quadratic effect on in vitro gas production (P < 0.05; 177, 181, 185, 190, and 170 mL for 0, 0.226, 2.26, 22.6, and 226.0 mg, respectively). Total VFA production was not significantly changed with RAC (P > 0.50). In Exp. 2, IVDMD was measured with tubes incubated with 0, 0.226, 2.26, or 22.6 mg of RAC/L of buffered ruminal fluid with 4 substrate combinations: corn, corn plus soybean meal, corn plus urea, and corn plus soybean meal plus urea. Dry matter disappearance was measured after 2, 4, 6, 8, or 12 h of fermentation. There was an interaction between RAC and substrate (P < 0.01), with more degradable forms of nitrogen eliciting greater IVDMD from RAC. Significant main effects also were detected for RAC, substrate, and hour (P < 0.001). In Exp. 3, AA and ammonia were measured in tubes treated with 0 or 2.26 mg of RAC/L of buffered ruminal fluid. Tubes were incubated for 0, 15, 30, 45, 60, 75, 90, 120, 150, 180, 210, or 240 min. There were decreases in ammonia and AA concentrations with RAC (P < 0.001). Experiment 4 used 16 ruminally fistulated Holstein steers in a 2 x 2 x 2 factorial arrangement of treatments. Factors consisted of grain processing method (steam-flaked or dry-rolled corn), concentration of dried distillers grains (DG) with solubles (0 or 25% DG, DM basis), and concentration of RAC (0 or 200 mg/d). Ruminal ammonia concentrations were less when RAC was fed in combination with dry-rolled corn, but not when RAC was fed in conjunction with steam-flaked corn (grain processing x RAC, P < 0.01). Addition of RAC, steam-flaked corn, and DG all resulted in reduced ruminal ammonia concentrations (P < 0.01). Amino acid concentrations were decreased when RAC was added to diets with DG but were unchanged in diets without added DG ( DG x RAC, P < 0.05). Changes in ruminal ammonia and AA concentrations with RAC supplementation are dependent on grain processing and the addition of DG to finishing diets. Results from these studies suggest that RAC affects fermentation by ruminal microflora. Effects of the interactions between RAC and protein source, grain processing, and DG on proteolysis could have important implications when formulating diets for cattle supplemented with RAC.