Genotype-by-environment interaction with broiler genotypes differing in growth rate: 2. The effects of high ambient temperature on dwarf versus normal broilers

High ambient temperatures (AT) reduce feed consumption (FC) and BW in broil ers, thereby leading to lower efficiency and profitability of poultry meat production in hot climates. These negative effects have been found to be mo re pronounced in chicken lines with high BW. The effects of high AT were in vestigated in a broiler population segregating for the Dw gene and, thus, c onsisting of normal-sized and dwarf broilers, which differed markedly in BW but had the same genetic background. All chicks were reared under normal A T (constant 22 C) up to Day 44, when AT was gradually raised, over 24 h, to 32 C and then held constant to Day 49. The dwarf chicks had 23% lower BW a nd BW gain (WG) at all ages until Day 44. During the first 24 h at 32 C, FC of the dwarf broilers was reduced by 35% and their average WG was 7.6 g/d, whereas FC of their normal-sized counterparts was reduced by 46% and they lost BW (average WG: -42.1 g/d). Thereafter, however, the dwarf and normal broilers adapted similarly to the chronic heat stress, with FC and WG of bo th phenotypes averaging 72 and 35%, respectively, of the corresponding mean s at normal AT. The two phenotypes had similar body temperatures at normal AT, but following its increase, body temperature in the normal-sized broile rs rose by 1.14 C, whereas in the dwarf ones it rose by only 0.47 C. This f inding suggested better thermoregulation during acute heat stress, apparent ly due to the latter's smaller body size. Under chronic heat stress, howeve r, FC and WG were similarly reduced in the dwarf and normal broilers. We co ncluded that the dwarf gene has no value with regards to broiler tolerance to chronic heat stress, either for production or as a model.