The swine estrous cycle is the sequence of hormonal and physiological events in sexually mature female pigs, known as sows and gilts, that prepares them for breeding and pregnancy. A thorough understanding of this recurring cycle allows producers to manage breeding schedules, improve conception rates, and maintain a consistent flow of animals through the production system.
Phases of the Estrous Cycle
The swine estrous cycle averages 21 days, ranging from 18 to 24 days, and has four phases. It begins with proestrus, a two- to three-day period where the hypothalamus releases gonadotropin-releasing hormone (GnRH). This signals the pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate the growth of multiple ovarian follicles.
Growing follicles produce increasing amounts of estrogen, triggering the next phase, estrus. This period of sexual receptivity, or “heat,” lasts 40 to 60 hours. High estrogen causes a massive LH surge, inducing ovulation 30 to 40 hours after estrus begins, with an average release of 15 to 24 eggs.
Following ovulation is metestrus, a phase lasting about two days. During this time, the ruptured follicles transform into structures called corpora lutea (CL). These newly formed CL begin producing the hormone progesterone, which ends sexual receptivity.
The final phase is diestrus, lasting 14 to 16 days. The corpora lutea actively secrete progesterone, which inhibits FSH and LH release, preventing new follicle growth. If not pregnant by day 16, the uterus releases prostaglandin F2α, causing the corpora lutea to regress so the cycle can restart.
Recognizing Estrus (Heat)
Identifying when a female pig is in estrus is necessary for successful breeding, especially with artificial insemination. The signs are driven by high estrogen levels and include physical and behavioral changes. One of the earliest physical signs is a reddened and swollen vulva, which can appear two to three days before she is ready to be bred, and may be accompanied by a clear, sticky mucous discharge.
Behavioral indicators become more apparent as the female nears peak receptivity. She may become restless, vocalize, mount other females, and show increased interest in the boar. Other signs can include pricked ears and a poor appetite.
The definitive sign of estrus is the standing reflex, or lordosis response. A female in standing heat will become immobile and arch her back when pressure is applied to her back and flanks. This response signifies she is receptive to mating and is most effectively stimulated by a mature boar, but can also be elicited by a person applying firm pressure.
Factors Influencing Cyclicity
The regularity of the estrous cycle is influenced by several external and internal factors. Age is a primary determinant, as gilts must reach puberty between 170 and 210 days of age before they begin cycling. Nutrition also plays a role; underfed gilts and sows may have delayed cycles, while increasing feed intake before breeding, known as “flushing,” can increase ovulation rates.
Environmental conditions can disrupt cyclicity. Heat stress during hot months is a common cause of reduced reproductive efficiency and can delay estrus. Social factors, such as stress from mixing unfamiliar animals, can also interrupt cycles, while the “boar effect” from a mature boar’s presence can induce puberty in gilts.
Lactation naturally suppresses the estrous cycle in sows. The physical stimulation of piglets suckling inhibits the release of reproductive hormones, a state known as lactational anestrus. The removal of piglets at weaning triggers a hormonal shift that initiates a new follicular phase, with most sows returning to estrus within four to seven days.
Managing the Cycle for Breeding
In modern swine production, managing the estrous cycle with estrus synchronization is a standard practice. This involves controlling the cycle so a large group of females comes into heat and can be bred simultaneously. This practice improves labor efficiency and results in weaned pigs that are uniform in age.
Hormonal products are the main tools used for this control. Progestogens, such as the oral product altrenogest, mimic the hormone progesterone. When fed to cycling females for a set period, it prevents them from coming into estrus; once the product is removed, the animals synchronously enter estrus within four to nine days.
For non-cycling gilts that have reached a mature age, a combination of gonadotropins can be used to induce a first heat. A common product, P.G. 600, contains equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG). These act similarly to FSH and LH to stimulate follicle development and ovulation.