Influence of the duration of daylight on prolactin secretion in the pig: hourly rhythm in ovariectomized females, monthly variation in domestic (male and female) and wild strains during the year.

In wild boars and sows, plasma prolactin varied among seasons with highest levels in July and lowest during the winter months. In domesticated breeds of pigs, plasma prolactin of intact boars did not vary among seasons but plasma prolactin of gilts tended to be higher in the summer than in winter. However, this difference was more conclusive in ovariectomized as compared to intact gilts. There was not a significant circadian rhythm in hourly samples from ovariectomized gilts.