Impact of heat stress on growth performance and carcass traits of fast- medium- and slow-growing broiler chicken genotypes.

The present study aimed to evaluate the effects of genotype, environmental temperature, and pen enrichment on growth performance and carcass traits in three broiler chicken genotypes. A total of 720 day-old male chickens belonging to three genetic lines differing from growth rate (fast-growing - FG; medium-growing - MG; and slow-growing - SG) were reared in 36 pens (3 m per pen; 20 birds per pen) across two separate rooms (18 pens per room) under either controlled environmental temperatures (22.8 ± 2.8°C) or heat stress (HS) conditions (27.2 ± 2.8°C) until slaughter (40 d for FG; 55 d for MG; and 76 d for SG). In half of the pens (18 in total; 9 per room), a perch was provided as environmental enrichment. FG broilers exhibited the highest daily weight gain, feed intake, and the most efficient feed conversion ratio, whereas MG and SG chickens showed intermediate and lowest performance levels, respectively (P < 0.001). MG chickens showed the highest slaughter yield but an intermediate breast yield, which was surpassed by that of FG broilers (P < 0.001). In the pectoralis major muscle, FG chickens displayed the highest values for ultimate pH, lightness, and yellowness, whereas SG birds showed the highest redness index (P < 0.001). HS conditions significantly reduced growth performance (P < 0.001) and slaughter yield (0.05 < P < 0.001) across all three genotypes, while increasing the incidence of wooden breast in FG chickens (P < 0.05), compared to those reared under controlled conditions. However, the three genotypes exhibited differential responses to HS, particularly in terms of growth performance. Indeed, the negative effects of HS were more pronounced in FG and MG chickens, whereas SG chickens demonstrated greater resilience to the same environmental challenge (0.05 < P < 0.001; significant Genotype × Temperature interaction). Providing a perch as environmental enrichment had no significant effect on broiler growth performance but significantly reduced the incidence of white striping in the breast fillets of FG chickens (P < 0.05). In conclusion, SG broilers showed the lowest growth performance and slaughter yield but exhibited greater resilience to chronic HS compared to FG and MG genotypes.