Why do some sows not come into heat for a long time after farrowing or fail to mate during estrus?

　　Long-term anestrus in sows, failure to mate during estrus, low litter size, weak physical condition, etc., are all signs of decreased reproductive capacity. The causes of decreased reproductive capacity are multifaceted, mainly including:

　　(1) The breeding pigs themselves have close inbreeding, or suffer from nutritional deficiencies during the breeding stage, or have chronic wasting diseases, poor growth and development, and underdeveloped reproductive organs.

　　(2) Feed factors: insufficient feed, nutritional deficiencies, especially lack of protein, minerals, and vitamins, leading to emaciation and suppression and damage of reproductive function; or adequate feed but improper composition, with excessive carbohydrate feed, such as long-term feeding of cassava or corn alone, especially with insufficient exercise, causing obesity in sows, fat deposition in the ovaries, fatty degeneration of follicular epithelium, impaired ovarian blood circulation, and difficulty in egg formation. Feeding breeding pigs with sour feeds such as spoiled and acidic swill, alcohol, cassava residue, tofu residue, etc., adversely affects reproductive function. Abuse of hormone-containing feed additives can cause sexual dysfunction or systemic edema, with corresponding edema of the uterine mucosa, making conception difficult.

　　(3) Improper use: frequent mating or semen collection from boars leads to diluted semen and reduced quality. Late weaning in sows causes nutritional substances to be lost through milk, resulting in insufficient nutrition for the reproductive system, and the suckling stimulation of piglets suppresses ovarian function via the pituitary gland. Breeding pigs that are too old, with sows over 3 years gradually aging, and after giving birth to the 6th litter at 3.5 years, reproductive capacity gradually declines. Older sows often have more diseases and poor recovery after illness, leading to deteriorating physical condition, reduced litter size, lower birth weight of piglets, and declining lactation performance.

　　(4) Environmental factors: cold or stuffy heat, lack of exercise and sunlight, noisy, chaotic, and dirty environments causing discomfort.

　　(5) Disease factors: infection with Brucellosis, Chlamydiosis, endometritis, and other chronic wasting diseases such as pleuropneumonia and parasitic diseases.

　　(6) Human factors: lack of training for sows, improper timing of mating—too early or too late—affecting fertilization, or long-term failure to mate leading to suppression of sexual function and decreased reproductive capacity.

　　Can pregnant sows be dewormed?

　　Deworming drugs generally have toxicity and can easily cause miscarriage in pregnant sows, so deworming is generally not done during pregnancy. However, levamisole has very low toxicity, and long-term experimental observation and clinical application have not found it to cause miscarriage, so deworming can be done in early pregnancy. For sows in late pregnancy, to be cautious, it is better to deworm after farrowing.