Interactions of dietary nutrient density with genotype and sex in broiler chickens: implications for growth, carcass...

This study aimed to assess the interactive effects of dietary nutrient density, broiler genotype, and sex on growth performance, breast meat yield and quality, nutrient digestibility and utilization in straight run broilers. A total of 1080 straight-run Ross 308 and 1080 straight-run Cobb 500 chicks were offered 3 dietary treatments formulated to either Ross 308, Cobb 500, or AgriFutures (AGF) nutrients specifications. AGF specifications reduced ME by 100 and 75 kcal/kg in starter and grower diets, and increased ME by 25 kcal/kg in withdrawal compared with Ross 308 guidelines. Each treatment was replicated 8 times with 45 birds per replicate, and fed for starter (0-10 d), grower (10-24 d), finisher (24-35 d), and withdrawal (35-42 d) phases. On day 35, six birds per pen (3 males, 3 females) were euthanized for digesta collection to determine nutrient digestibility; on day 42, a further six birds per pen (3 males, 3 females) were euthanized for carcass yield and breast meat quality assessments. Final BW on day 42 was not affected by nutrient density, however, as a main effect, Cobb birds exhibited faster growth rate and achieved higher BW but also had a higher FCR than Ross (P < 0.01). In addition, as a main effect Cobb specification resulted in higher FI and FCR (P < 0.01). Despite the higher ME and digestible lysine intake and higher nutrient conversion ratio in Cobb birds (P < 0.01), there were no significant differences in nutrient digestibility between the two genotypes (P > 0.05). Sex significantly influenced weight gain with female birds exhibiting lower BW and higher fat deposition (P < 0.01). Female birds also had lower digestibility of protein, energy and fat compared to males (P < 0.01). The present study showed that AGF specifications with reduced dietary ME by 100 and 75 kcal/kg in the starter and grower diets respectively, does not compromise growth performance in straight-run broiler chicks. Cobb broilers responded positively to increased dietary amino acid density (Ross and AGF specifications), which was reflected in lower FCR.