Gilts are selected for breeding on growth rate, feed conversion and reproductive history of their dams and sires. Also their own anatomy. They used to not be bred at first oestrus, yet intensive production has changed this and nowadays they are mated as early as possible. Contact with a male by sight, sound or smell triggers first oestrus. Gilts who fail to come into heat by eight months are killed.
Some farms synchronise oestrus in batches of sows by giving hormones. Gonadotropins stimulate oestrus and progestogens may also be used – synchronisation is adopted to avoid costs of staff being paid at night or weekends, and to balance up litters.