Why don’t all sows return to heat within 4 to 7 days after they are weaned?
Sows are too thin when weaned and they do not have sufficient body stores of fat to allow them to become pregnant. This is related back to the hypothalamus and the secretion and release of GnRH. This is particularly a problem with young, P1 and P2 sows and those sows which are early weaned (prior to day 16 of lactation).
Some farms experience a seasonal problem, usually, during hot weather and with younger sows.
Sows which experience “dystocia” or difficult farrowing may have a delayed return to esrus.