Estrus in Swine

| Biology Basics | Estrus synchronization | Signs of Estrus | Estrus Detection | Refractory Sows |

Importance of Estrus

Understanding and managing estrus in swine is important because estrus may be thought of as the starting point of the swine herd life cycle. Behavioral estrus lasts 2-3 days and ovulation occurs near the end of behavioral estrus. Knowing when estrus will occur, detecting it, and inseminating the female 12-26 hours after onset of standing heat (estrus) so that the sperm are deposited before ovulation will ensure high conception rates and low repeat breeders.

Effect of time of breeding on pregnancy rate in sows.

The Biology Basics
  • The sow is polyestrous, with a period of lactational anestrus occurring until after weaning.
  • The estrus cycle length averages 21 days.
  • Return to estrus occurs after the recovery period, or 4-7 days post-weaning
  • Estrus onset is marked by the preovulatory surge of LH. Gilts come in to first estrus around 8 months of age. 
  • Estrus (heat behavior) occurs for 2-3 days, averaging 60 hours in mature sows, but is only about 48 hours long in gilts.
  • Ovulation occurs during the last third of estrus (on diagram at top).
Managing the Basics

Estrus Synchronization Estrus onset is influenced by energy balance and nutrition during lactation. Weaning is used as an estrus synchronizing event in swine. When sows go into heat together, and conceive together, they will farrow their piglets together in groups which make all in/all out flow possible. 

Signs of Estrus

  • The most definitive behavioral sign of estrus is standing to be mounted by the boar. 
  • Sows in estrus will often assume this rigid stance, called the lordosis reflex, when pressure is applied on the rump ("back-pressure") by the herdsman.
  • The group-housed sow actively seeks out the boar. 
  • The vulvar lips are swollen and red with a thin, mucous discharge. 
  • Other signs of estrus include: depressed appetite, restlessness, alertness, pacing, grunting, and chomping of the jaws.

Estrus Detection Deficient estrus detection is the most important cause of infertility in breeding herds using hand-mating or artificial insemination (AI) systems. Typically, sows are checked for estrus once a day and gilts twice a day. In herds with estrus detection problems, heat checking sows twice a day is recommended. Estrus detection can be improved by observing sow behavior while the boar is given direct contact with the sow.

Refractory Sows However, sows and gilts will become fatigued and refractory to boar contact in less than one hour, even if they are in heat. This is due in part to the extreme exertion (isometric contraction) associated with standing heat. Thus, best estrus detection systems do not allow constant boar contact. For estrus detection in the absence of a boar, response to the "back-pressure" test can be potentiated by sex-odor sprays (ex. Boar-mate) or tape recordings of a boar. 

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Copyright ©1997 by the University of Pennsylvania School of Veterinary Medicine
Faculty: Dr. Paul Pitcher
Student: Sandra Springer,'99