Assessing the thermo-tolerance potentials of five commercial layer chicken genotypes under long-term heat stress environment as measured by their performance traits This study was conducted to test ...the thermo-tolerance ability of five commercial chicken genotypes (Lohmann Brown, LB; Lohmann White, LW; New Hampshire, NH ; White Leghorn selected for low feed expenditure, WL-FE and White Leghorn with sex-linked dwarf gene, WL-dw) under long-term heat exposure. Two-hundred forty female chickens were assigned to a completely randomized design in a 5 × 2 factorial arrangements (five genetic groups and two ambient temperatures thermo-neutral, 18-20 °C; heat stress, 30-32 °C). Individual eggs were collected on daily basis while egg weight and feed intake were determined on individual and group basis at 28-days intervals, respectively. Shell quality traits were determined at 25, 40 and 56 weeks age. No Genotype × ambient temperature interactions were found except for body weight and egg deformation. Chickens at thermo-neutral temperature produced significantly heavier eggs than those of heat-exposed (60 g vs. 54 g). Hen-housed egg production of chickens in thermo-neutral temperature was significantly higher than those of heat-stressed (76.8 % vs. 66.2 %). Daily egg mass production at thermo-neutral and heat stressed chickens was 46 g and 35.8 g, respectively. Feed consumption in heat-stressed and thermo-neutral chickens was 109 and 80.8 g, respectively. Shell thickness, breaking strength and Haugh unit values were significantly reduced in heat-stressed chickens. Among heat-exposed chickens, the NH had the highest body weight while the LW produced 10 % more eggs than the group average. The heat-induced effect on shell quality traits was lowest in LW chickens. The results indicated that the magnitude of heat stress was breed dependent in which the LB showed poor adaptability to heat stress while both NH and LW genotypes demonstrated better thermo-tolerance ability.
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