What are the methods for identifying estrus in sows?

    Early estrus: The sow's vulva begins to swell and reaches peak swelling. At this time she is restless, chases and mounts other pigs, and is not still when you press down on her back with your hand. 

Mid-estrus: The vulvar swelling begins to subside, slight wrinkles appear, the sow stares blankly, urinates frequently, stands still when you press down on her back with your hand, a white viscous fluid flows from the vagina, and she accepts the boar mounting her and allows mating. 

Late estrus: The vulvar swelling gradually subsides until it disappears completely, with a white frost. She does not allow the boar to mount or mate.