Effect of dietary methionine on performance, carcase characteristics and breast meat composition of heterozygous naked neck (Na/na+) birds under spring and summer conditions

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Abstract
1. Heteroxygous naked neck (Na/na+) birds and their normally feathered counterparts (na+/na+) were fed from 0 to 7 weeks on 3 diets differing in methionine concentrations. From 0 to 3 and 3 to 7 weeks, respectively, the concentrations were: low containing 4.3 and 3.3 g/kg; optimum containing 5.0 and 3.8 g/kg and; high with 5.7 and 4.4 g/kg under spring (optimum ambient temperature) and summer conditions (high ambient temperature). Performance, carcase characteristics and breast meat chemical composition were determined. 2. Summer rearing resulted in a decrease in body weight, body weight gain, food consumption, and yields of carcase and breast. The summer temperature effect was more pronounced in males. Under summer temperatures, the protein content of the breast decreased while the fat content increased compared to birds reared in spring. 3. By 7 weeks of age, both genotypes reached similar body weights in the spring experiment while, in summer Na/na+ birds were 3.3% heavier and gained more in the period from 3 to 7 weeks than na+/na+ birds. Carcase and breast yields of Na/na+ birds were greater than in na+/na+ birds. 3. Second order polynomial coefficients of the dietary methionine effect were found to be significant for body weight at 3 and 7 weeks. Daily body weight gain between 3 and 7 weeks was linearly affected by the dietary methionine concentration. There was no interaction between genotype and methionine. 4. Methionine had no significant effect on carcase yield. Second order polynomial coefficients of the dietary methionine effect were found to be significant for breast yield while the methionine effect on abdominal fat was linear. Na/na+ females fed on the low methionine diet had lower protein content than the Na/na+ males. 5. It is concluded that the methionine requirement of Na/na+ birds did not differ from that of their normally feathered counterparts under either spring or summer ambient temperature conditions.

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