Low-intensity, short-duration thermal stimulation during the late phase of incubation alters secondary sex ratio in favour of males.

1. In two experiments, two setters and hatchers, with a capacity of 42 240 eggs each, were used to investigate the effect of low-intensity, short-duration thermal stimuli during the late phase of incubation on hatchability, sex ratio and grow-out performance of broilers under field conditions. 2. Eggs in the test group had the same physical environment as eggs in the control group except that incubation temperature was increased by 0.5°C for 2 h/d above the control group from 18 to 20 d of incubation. 3. Thermal stimulation significantly increased the proportion of males hatched in both experiments. In experiment 2, evaluation at 7 d of age showed that the proportion of males in the test group was still significantly higher than in the control group. 4. In experiment 2, hatch residue was examined and the proportion of unhatched male embryos was significantly greater in the control group than in the test group. 5. Thermal stimulation did not have a significant influence on post-hatch performance of broiler chickens to market age. 6. The results demonstrated that thermal stimulation of 0.5°C for 2 h/d above the control during late incubation shifted the sex ratio at hatch and at 7 d in favour of males. The difference in secondary sex ratio was due to increased survival of male embryos in the test group.