| Abstrakt: |
Inclusion of distillers grains (DG) in cattle diets has been shown to increase fecal shedding of Escherichia coliO157: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. coliO157: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. coliO157: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. coliO157:H7. Isolation of E. coliO157: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 stx2were considered positive for E. coliO157: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. coliO157: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. coliO157: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. coliO157:H7. Fecal prevalence of E. coliO157: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. coliO157:H7 (4.4 vs. 4.0%). Additional research is needed to confirm the reduction in fecal shedding of E. coliO157:H7 in cattle fed monensin at 44 mg/kg of feed compared with cattle fed 33 mg/kg of feed. |
