The successful management of a swine breeding program relies on a precise understanding of the female’s reproductive cycle. This biological process, known as the estrous cycle, governs the window of opportunity for conception in sows and gilts. Mastering the timeline and physical cues associated with this cycle is fundamental to maximizing reproductive efficiency and litter size. Effective breeding requires carefully timed action based on recognizing specific physiological signals.
Defining the Swine Estrous Cycle
The reproductive cycle in pigs is continuous (polyestrous) and not limited by a specific breeding season. The cycle length averages 21 days, though it can vary naturally from 18 to 24 days. This hormonal process prepares the female for ovulation and potential pregnancy.
The cycle is divided into four distinct phases. Proestrus is the first phase, where ovarian follicles begin to develop, driven by rising estrogen levels. This phase prepares the female for heat and leads directly into the next stage.
Estrus, commonly called “heat,” is the period of sexual receptivity, generally lasting 40 to 72 hours. Estrogen levels peak during this phase, causing behavioral changes that indicate the female is ready for mating. Ovulation, the release of eggs, typically occurs late in this phase, about two-thirds of the way through the estrus period.
Following estrus is Metestrus, a short transition where ruptured follicles transform into corpora lutea (CL). The CL becomes fully functional during Diestrus, the final and longest phase (13 to 15 days). Progesterone dominates Diestrus, suppressing new follicle development and maintaining the uterine lining. If pregnancy does not occur, the CL regresses, progesterone levels drop, and the cycle restarts.
Recognizing Estrus
Identifying the onset of estrus is the most important practical skill in swine breeding management. The period of heat is defined by changes in behavior and physical appearance, driven by high estrogen levels. Standing heat duration is variable, lasting around 38 to 48 hours in gilts and up to 72 hours in mature sows.
The most definitive sign of true estrus is the standing reflex. A female exhibiting this reflex will stand rigid and immobile when pressure is applied to her back, indicating she is willing to accept mating. This response is confirmed by a stockperson applying firm pressure to the loin area or gently sitting astride the female.
Signs of Estrus
Other observable signs often accompany the standing reflex:
- Swollen and reddened vulva (more pronounced in gilts).
- Increased restlessness.
- Heightened vocalization.
- Mounting penmates.
Estrus detection is enhanced by boar exposure, as the sight, sound, and smell of a mature boar intensify the heat signs and help trigger the standing reflex. It is standard practice to check for estrus twice daily, ideally 8 to 12 hours apart, to accurately pinpoint the onset of heat.
Timing and Reproductive Management
Optimizing the timing of insemination relative to ovulation is central to successful breeding. Since eggs have a short lifespan after release, the goal is to ensure highly fertile sperm are present when ovulation occurs. Ovulation happens approximately 30 to 40 hours after the onset of standing estrus, meaning the optimal breeding window is 12 to 24 hours before ovulation.
For artificial insemination (AI), precise timing uses the “AM/PM rule” based on twice-daily heat checks. A female found in standing heat in the morning (AM) is inseminated that evening (PM), and one found in the evening is inseminated the following morning. This protocol aims to place sperm in the reproductive tract approximately 12 hours after initial detection.
Multiple inseminations are often performed to maximize conception rates, especially in sows where estrus duration is longer. A common practice is to inseminate a sow twice, 12 to 24 hours apart, provided she maintains the standing reflex. The timing is often adjusted based on the female’s age, with gilts typically having a shorter estrus and requiring an earlier first insemination compared to mature sows.
Estrus in Weaned Sows
In weaned sows, the return to estrus is a predictable management point, usually occurring 4 to 7 days after the litter is removed. Managing nutrition and body condition during lactation is important, as excessive weight loss can delay this return, leading to non-productive days.