EFFECT OF GROWTH PROMOTERS ON MONENSIN TOXICITY IN BROILER CHICKS

1. An experiment was carried out with male broiler chicks to evaluate the combined effect of monensin (150 mg/kg) and the growth promoters ( GPs) Zn bacitracin (BAC, 50 mg/kg), virginiamycim (VIR, 25 mg/kg) and avoparcin (AVO, 20 mg/kg) fed from 7 to 28 d of age on performance, ut ilisation of dietary nutrients, yield of defeathered eviscerated carca ses (DEC) and size of various organs. The effect of the GPs in the mon ensin-unsupplemented diets fed up to 49 d of age on performance and ca rcase was also determined. 2. Monensin significantly (P < 0.05) depres sed food intake, weight gain and food efficiency from 7 to 28 d of age . None of the GPs was able to counteract these effects. However, AVO s lightly ameliorated them. AVO also significantly increased food intake and improved gain and food efficiency during 7 to 28, but not 28 to 4 9 or 7 to 49 d of age. VIR and BAC did not affect performance in eithe r age period. 3. Monensin did not affect the utilisation of dietary dr y matter, fat or energy, but it significantly decreased nitrogen utili sation. AVO improved nitrogen and fat utilisation and increased dietar y AME(n) content. AME(n) was also increased by VIR. The utilisation of these nutrients was not affected by the interactions between monensin and the GPs. 4. Monensin did not affect yield of the DEC or the relat ive liver size at 31 d of age. It significantly increased the relative length of the small intestine (SI) and decreased its specific weight. AVO significantly increased yield at 31, but not at 53 d of age. BAC and VIR did not affect this variable. AVO and VIR, but not BAC, at bot h age periods reduced, at times significantly, the size, length and sp ecific weight of the SI. 5. Our conclusions: BAC, VIR and AVO do not c ounteract the toxic effect of monensin. The effect of GPs in improving performance decreases and even disappears with age, while their effec t in reducing the size of the SI is still evident in 49-d-old birds.