External effectors of reproduction include photoperiod (day length), pheromones, temperature, nutrition, and stress. Responses to these effectors are communicated to the hypothalamus, where the secretion of releasing and inhibiting hormones, specifically GnRH and dopamine, is controlled. Photoperiod affects reproduction in all mammals and birds. However, some animals (e.g., sheep, goats, and poultry) are extremely sensitive to changes in day length. The annual initiation of the estrous cycle in sheep and goats is dependent on decreasing day length so that sheep and goats are called short-day breeders. Poultry need 14 17 hours of daylight for maximum egg production.
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