Lack of effect of dietary energy-to-protein ratio and energy concentration on the response of broiler chickens to virginiamycin.

1. Two factorial experiments were carried out with male broiler chicks from 6 to 28 (experiment 1) and from 7 to 49 (experiment 2) d of age to evaluate the combined effect of virginiamycin (VIR), dietary energy concentration, and energy-to-protein (E:P) ratio on performance and the utilisation of dietary nutrients. 2. VIR had no significant effect in either experiment on food intake or weight gain, but significantly improved food efficiency up to 28 d of age (P less than 0.001 and P less than 0.05 in experiments 1 and 2, respectively). 3. VIR consistently and significantly (P less than 0.01) decreased fat excretion, improved fat relative retention and increased dietary AMEn. VIR also significantly (P less than 0.01) increased relative retention of dry matter, while it did not affect nitrogen balance. 4. Neither of the variables of performance and utilisation of dietary nutrients was affected by the interactions between VIR and either dietary E:P ratio or energy concentration. It is concluded, therefore, that the effect of VIR in increasing the utilisation of dietary energy does not depend on the above factors.