An understanding of the sow’s reproductive system is essential for a successful mating program, whether AI or natural service is used. This discussion will be divided into three areas: (1) Anatomy of the reproductive tract, (2) estrous cycle, estrus and ovulation, and (3) embryonic and fetal development.
Anatomy
The primary structures of the female reproductive tract are the ovaries, they have two major functions:
(1) to produce ova, the female germ cells and
(2) to produce the hormones progesterone and estrogen.
Each ovary is surrounded by a thin membrane called the infundibulum which acts as a funnel to collect ova and divert them to the oviduct. The oviduct is about 6-10 inches long and acts as the site of fertilization.
There are two uterine horns. Each is 2-3 feet in length in the non-pregnant sow. They act as a passageway for sperm to reach the oviduct and are the site of fetal development. The uterine body, which is small compared to some other species, is located at the junction of the two uterine horns.
The cervix is a muscular junction between the vagina and uteri. It is the site of semen deposition during natural mating and AI. It is dilated during heat (estrus) but constricted during the remainder of the estrous cycle and during pregnancy.
The vagina extends from the cervix to the vulva and serves as a passageway for urine and the piglets at birth. The bladder is connected to the vagina by the urethra.
The vulva is the external portion of the reproductive tract. It often becomes red and swollen just prior to estrus and this swollen condition is usually more pronounced in gilts than in sows.
The hypothalamus located at the base of the brain secretes gonadotropin releasing hormone (GnRH) which regulates the anterior pituitary gland to secret FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone) into the blood which stimulates the production of the ovarian hormones, estrogen and progesterone, which in turn regulate the reproductive process. Oxytocin is released from the posterior pituitary gland.
Estrous Cycle, Estrus and Ovulation
Non-pregnant and non-lactating sows and gilts display estrus or standing heat on a regular basis throughout the year. The estrous cycle is normally 21 days and is defined as the time between the onset of one estrus to the onset of the next. The cycle length can range from 18-24 days.
Lactation or the nursing stimulus inhibits the estrous cycle and sows will not, as a rule, return to heat until the litter is weaned. Days from weaning until estrus is influenced by such factors as length of lactation, parity, season and nutritional level, but should range from 4 to 7 days.
As estrus or heat approaches, 6-10 follicles or "blister like" structures form on each ovary. Follicular growth accelerates about 3 days before estrus and is influenced by FSH or follicle stimulating hormone released from the pituitary gland, located at the base of the brain. A maturing ovum is held within each follicle. Granulosa cells within the follicle secrete estrogen, a hormone, which among other things is responsible for the typical signs of estrus. Ovulation, or release of the ova, is stimulated by LH. Ovulation occurs about 40 hours after the onset of estrus, but this interval is variable.
Several factors can influence ovulation rate or number of ova shed.
1. Age. Sows may ovulate 18-20 ova while gilts may ovulate 12-14 ova.
2. Nutrition. Flushing (increased energy levels prior to estrus) may increase ovulation rate yet may have little effect on the ultimate litter size.
3. Breed. The white or maternal breeds generally have a higher ovulation rate. Crossbred females generally have a higher ovulation rate than either of the parent breeds.
The onset and disappearance of estrus and estrus behavior is gradual and there are individual differences among females (see Fig. 3). The primary sign, and most reliable sign, of estrus is "standing" while another sow or the boar mounts. Many females will stand for the "back pressure test" when applied by the herdsperson. A higher percentage of females will respond to the "back pressure test" if there is a boar present. Therefore, use of an intact or a vasectomized boar is an important part of a regular heat detection program. Boars secrete pheromones (odors) in their salivary glands which elicit the standing reflex of the female. Mature boars are superior to young boars in stimulating this response.
Secondary signs of estrus include:
o Red, swollen vulva which is usually more pronounced in gilts than in sows.
o Increased nervous activity.
o Desire to seek the boar.
o Loss of appetite.
o Male-like sexual behavior (pursuing, nosing and mounting other females).
o Change in vocalization (grunts and growls).
o Increase in vaginal mucous (thumb check).
By Dr. Wayne Singleton and Dr. Mark Diekman
Purdue University Department of Animal Sciences