Effects of tylosin on concentrations of Fusobacterium necrophorum and fermentation products in the rumen of cattle fed a high-concentrate diet

Objective-To determine effects of tylosin on ruminal concentrations of Fuso bacterium necrophorum and fermentation products in cattle during rapid adap tation to a high-concentrate diet. Animals-6 steers fitted with ruminal cannulas. Procedure-Steers were assigned randomly to 2 treatment groups and switched from a 0 to an 85% concentrate diet during a 4-day period. Cattle received this diet, with or without tylosin (90 mg/steer/d), for 4 weeks. Samples of ruminal contents were collected daily beginning 2 days before the treatmen t protocol and in the first week of concentrate feeding. Four subsequent sa mples were collected at weekly intervals. Concentration of F necrophorum in samples was determined, using the most-probable-number technique. Ruminal pH and concentrations of volatile fatty acids (VFA), lactate, and ammonia a lso were determined. Ail steers received both treatments separated by 4 wee ks (cross-over design), during which time they were fed alfalfa hay only. Results-In control steers, concentration of F necrophorum increased in resp onse to the high-concentrate diet. Tylosin-fed steers had lower concentrati ons of F necrophorum than control steers at all times during concentrate fe eding. However, ruminal pH and concentrations of lactate, VFA, and ammonia did not differ between treatment groups. Conclusions and Clinical Relevance-Tylosin caused a significant reduction i n ruminal concentrations of F necrophorum during rapid adaptation to a high -concentrate diet but had no effect on fermentation products. The reduction in ruminal concentration of F necrophorum helps explain the reduction in p revalence of hepatic abscesses reported in tylosin-fed feedlot cattle.