Effects of energy concentration and duration of feeding on the response of broiler chicks to growth promoters1

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Abstract
1. Two experiments were carried out with male broiler chicks from 7 to 49 d of age to evaluate the combined effect of the growth promoters (GPs) avoparcin (AVO, 20 mg/kg), Zn bacitracin (BAC, 25 mg/kg) and nitrovin (NIT, 18 mg/kg), and dietary energy concentration (EC) (experiment 1) and the effect of age of feeding AVO (experiment 2) on performance, utilisation of dietary nutrients, yield of defeathered eviscerated carcases (DEC) and size of various organs. 2. AVO, in contrast to BAC and NIT, significantly (Pn content, weight gain and food efficiency. AVO also significantly decreased the size of the liver and the small intestine (SI), the length of the SI and its specific weight. The effect of BAC and NIT on the variables of the SI was less pronounced than that of AVO. Although the effect of AVO on increasing fat retention, growth and food efficiency was more pronounced with the high EC diet, neither of the variables of performance, nutrient utilisation (except for fat retention) or organ size, was significantly affected by the interactions between GPs and dietary EC. 3. AVO fed continuously or from 28 d of age significantly improved food efficiency during 7 to 49 d of age, increased the yield of DEC and decreased the size of the liver and the SI, the length of the SI, and its specific weight. None of these effects was observed when AVO was omitted from the diet at 28 d of age. Feeding AVO from 28 d of age did not improve weight gain during the entire period, as did the continuous feeding of this GP. 4. It is concluded that BAC and NIT are not effective as GPs under the conditions of the present study. The effect of AVO on improving performance and on carcase characteristics is hardly dependent on dietary EC. Feeding AVO from 28 to 49 d of age improves food efficiency and has similar effects on carcase characteristics to those of continuous feeding.