The average estrus cycle is 21 days (19-23 days). The estrus cycle can be divided into pre-estrus, estrus, late estrus, and intercourse. The characteristics of each period are outlined below:
1. Early stage of estrus: The sows move uneasy, the vulva swells, and changes from pale yellow to red. This change is more pronounced in gilts; the vagina has mucus secretion, and its viscosity gradually increases. During this period, sows are not allowed to ride on their backs. On average, they can have 2.7 days. Insemination should not be used for breeding.
2. Estrus: On average about 2.5 days, characterized by swollen pubic genitals and red beginning to decline, secretions are thickened, and viscosity increased. At this time, the sow is allowed to press back without moving. When the back is pressed, the sow's ears are erected backwards and the hind limbs are tight.
3. Late stage of estrus: 1-2 days, the genitals of the estrus sow completely return to normal, not allowing the boar to cross;
4. Intercourse period: 13-14 days, completely returned to normal. Sows can only allow boars to cross or call a standing response during estrus. This sexual behavior can be used as an indicator of timely insemination of sows. Sows with estrous within 4 days of weaning (fronted estrus sows) typically begin ovulation 60-72 hours after a standing reaction, 30-45 minutes to reach the site of fertilization, and ovulation lasts 4-6 hours. Sows that are weaned for 5-6 days (normal estrus sows) generally begin ovulation 40-56 hours after a standing reaction, 30-45 minutes to reach the site of fertilization, and ovulation lasts 4-6 hours. Sows who are weaned for more than 7 days (lagging estrus sows) and gilts generally begin to ovulate 16–28 hours after a standing reaction, 30–45 minutes to reach the site of fertilization, and ovulation lasts 4–6 hours. Pressback reflex is the highest peak of sows' estrus, which means that the ovulation time of sows is approaching. During this period, breeding can increase the fertility rate and litter size of sows.
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