We investigated the effect of different ambient temperatures and relative humidity (RH) with the same temperature-humidity indices (THI) on laying performance, egg quality, heterophil to lymphocyte ratio (H/L ratio), corticosterone (CORT) concentration in blood, yolk, and albumen, and plasma biochemical parameters of laying hens. Commercial hens (Hy-Line Brown; n = 120), aged 60 weeks, were allocated to two environmental chambers. Laying hens were subjected to either one of two thermal treatments-26 °C and 70% RH (TLHH75) or 30 °C and 30% RH (THHL75) for 28 days-with the same THI of 75. Neither TLHH75 nor THHL75 affected laying performance, including egg production, egg weight, egg mass, feed intake, and feed conversion ratio (p > 0.05). Plasma biochemical parameters such as total cholesterol, high-density lipoprotein cholesterol, triglycerides, calcium, magnesium, and phosphorus were not altered by the environmental treatments (p > 0.05). As for stress indicators, both environmental regimes failed to affect blood H/L ratio and CORT levels in plasma, yolk, and albumen (p > 0.05), although albumen CORT levels were elevated (p < 0.05) in TLHH75 group at day 7. Hence, our study suggests that laying hens performed and responded similarly when exposed to either TLHH75 or THHL75 characterized by the same THI. These results can serve as a scientific basis for management decisions and handling laying hens under thermally challenging conditions.
Keywords: egg quality; heat stress; laying hens; laying performance; stress indicators; temperature-humidity index.