The effects of environmental heat stress on reproduction in chickens were reviewed to find out the associated physiological changes and the interplay of the physiological events, after which the role of certain measures in the management of heat stress were identified. The normal rectal temperature for chicken is 40.6 - 43.0oC with 45oC as the upper limit of safety. The rectal temperature begins to increase when the ambient temperature increases, but this is prevented by panting to increase heat loss by respiratory evaporation of water. Chickens are said to be heat stressed when their body heat builds up during high ambient temperature. The physiological responses associated with it include respiratory alkalosis from panting, reduced feed intake and efficiency, reduced absorption of calcium, decrease in secretion of thyroid hormones, estrogen, progesterone and testosterone, and increased secretion of glucocorticoids, luteinising hormone and follicle stimulating hormone. Consequently, heat stress produces decrease in anabolism, increase in catabolism and insulin resistance, loss in body weight, delayed sexual maturity, reduction in number and quality of eggs, infertility in male and female chickens and reduced hatchability of fertile eggs with the greatest effect being on the number and quality of eggs produced. The impact of environmental heat stress remains a challenge as it affects reproduction in chickens and researches in this area are necessary to support increased productivity of chickens in hot semi-arid regions.
Key words: Chickens, eggs, heat stress, management, reproduction