Swine Production and Management | Home Study

Cycles and Heat Determination

Sows and gilts have an average 21 day heat cycle, although this can range from 17 to 25 days. An average animal in heat today will be in heat again in three weeks. This can be used to help synchronize farrowing dates in small herds so that animals are farrowing around the same time, and can be weaned of piglets (and therefore bred for their next litter) at the same time.

The first sign of a sow or gilt coming into heat is a red, swollen vulva.  Photo by Kaytee Norris and Emily McConnell

The first sign of a sow or gilt coming into heat is a red, swollen vulva. Photo by Kaytee Norris and Emily McConnell

Gestation length of the pig is 114 days, so average farrowing will occur 114 days after breeding. An easy way to remember this is “3 months (90 days), 3 weeks (21 days), 3 days” (90 + 21 + 3 = 114). When plotting this on a calendar be aware that adjustments are needed for months that are not exactly 30 days long.

After weaning, sows will come into heat again as quickly as 3 days but 4-7 days is more typical with an average return to heat of 5 days after weaning. Gilts are not so predictable and must be checked for heat daily until a standing heat is determined.

Heat detection and breeding timing are very important. Becoming familiar with swine behavior is essential to this critical management skill. Knowing both non-heat behavior and heat behavior makes it easier to identify when a female is coming into heat and when they reach standing heat. Ideally, breeding occurs during a time called ‘standing heat’ during which time the female will ‘lock up’, meaning that she will exhibit a natural behavior of standing rigidly and be receptive to mounting of a boar.

In the days prior to standing heat the female may or may not present physical signals that heat is coming. Look for changes in the female’s vulva. Vulvas of pigs nearing heat may be swollen, red, and discharge mucus. Applying pressure to the back of the animal to see if she will stand rigidly is the most common manner to check for standing heat. Often the female’s ears will be in an erect position during standing heat. Her tail may also move to expose her vulva.

Having an active, mature boar to help with heat checking is the best way to determine standing heat, but is not possible at every farm, especially when Artificial Insemination (AI) is practiced. Boar exposure brings out natural reactions that managers alone cannot reproduce. Nose-to-nose exposure to a boar will introduce the female to pheromones in the boar’s saliva and greatly enhance heat determination and breeding success.

A common reason for failure is breeding females before they are in a true standing heat. Take notes on each sow as they begin to exhibit signs of heat and check them every 12 hours until true standing heat is attained. Standing heat can last two days and breeding can occur every 12 hours during this time. With proper heat detection two or three matings are possible. However, a single mating at the correct time is better than multiple matings at the wrong time.

At farms utilizing AI, nose-to-nose contact can be a great tool to boost heat detection and breeding success. It is wise to keep the animals separated by a fence or other barrier so that AI can be conducted without interference from the boar. A barrier between the boar and both the sow and worker limits injury risk for all involved. When utilizing a boar to check for heat it is recommended that the animals be housed away from each other and brought together twice daily. This periodic exposure assures that heat responses occur during manager observation and not when you are away from the pigs. If no boar is located on the farm, commercial aerosol sprays that mimic boar odor are available.

Farms that utilize boars to breed the animals (non-AI) should also limit exposure time to twice daily. This allows manager observation and oversight of natural matings. Females that are receptive to the boar are most certainly in heat, but constant mingling of the animals can lead to female injury due to aggressive maneuvers of the often-larger boar. Giving the female a break between breeding services is good management.

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