Intended reduction in formulated net energy density and associated changes in diet composition improved growth performance and breast meat yield in broiler chickens under both cyclic heat stress and thermoneutral conditions.

This experiment investigated the effects of diets formulated to two net energy densities (2360 versus 2260 kcal/kg) and the impacts of cyclic heat-stress on the performance of broiler chickens via a 2 × 2 factorial array of treatments. Cyclic heat-stress consisted of the daily exposure of birds to an environmental temperature of 33°C for 8 hours as opposed to thermo-neutral conditions. Parameters assessed included growth performance from 14 to 35 days post-hatch, carcass traits, breast-meat myopathies, jejunal and ileal digestibility coefficients of starch, gross energy and protein (N). Cyclic heat stress significantly compromised weight gain, feed intake and FCR but did not influence breast-meat yields and myopathies, relative fat pad weights or jejunal and ileal digestibility coefficients for starch, protein and gross energy. The reduction in formulated dietary net energy improved FCR by 2.07% (P = 0.018) and increased breast-meat yields by 11.3% (P = 0.003). Millrun was included in the diets at 109 g/kg, and dietary crude protein was reduced, non-bound amino acids were included to achieve the intended reduction in formulated NE density. As this reduction in NE was achieved through concurrent changes in diet composition, the observed responses cannot be attributed to dietary NE alone. Importantly, these dietary modifications are inherent to practical NE-based feed formulation strategies and highlight the need for a greater understanding of the roles of dietary fibre or non-starch polysaccharides in influencing growth performance and carcass traits in broiler chickens.