Response to virginiamycin in finishing beef bulls fed a maize silage diet or a complete dry feed.

Two experiments have been conducted to study the effect of virginiamycin in a maize silage diet (I) or a complete dry feed (II). Virginiamycin was incorporated at 0 or 65 mg per kg concentrate which was fed at 0.75% of the live weight of 43 Belgian white-red bulls (I), or at 0 or 25 mg per kg complete dry feed for 24 Belgian white-blue bulls (II). In the first experiment daily gain was increased by virginiamycin from 1.28 to 1.38 kg without effect on intake, but with an improved feed conversion from 5.99 to 5.60 kg dry matter/kg gain. In the second experiment daily gain, daily dry matter intake and feed conversion were not influenced by virginiamycin. The differences between experiments may suggest that the response to virginiamycin probably depends on the diet type, although no significant interaction was found between diet type and virginiamycin with regard to growth rate (P greater than 0.10). In both experiments, fasting weight loss, dressing percent and carcass classification and composition were not affected by virginiamycin.