Effect of virginiamycin on in vivo digestibility, rumen fermentation and nitrogen balance

Three experiments were conducted to investigate the effect of virginiamycin. Digestibility was determined with wethers, fed a diet of 0.65 maize silage and 0.35 concentrate on a dry matter basis (Experiment I) or a complete dry feed (Experiment II) at maintenance, and with growing bulls fed a maize silage diet ad libitum supplemented with 7.5 g concentrate per kg live weight (Experiment III). Virginiamycin was incorporated at 0 or 65 ppm in the concentrate and 0 or 25 ppm in the complete dry feed. No significant effect on dry or organic matter digestibility was observed, although digestibilities of protein and ether extract were reduced by virginiamycin in experiments I and II, respectively. Rumen fermentation was studied in experiments I and II. Virginiamycin increased acetic acid concentration and reduced butyric acid concentration in experiment I, but exerted no significant effect in experiment II. An interaction between diet type and virginiamycin was found for the C2/C3 ratio. Nitrogen balance, measured in experiment III, was not affected by virginiamycin.