Reproductive success is measured primarily by pigs produced per sow per year and is dependent upon both far rowing rate and litter size. In order to achieve optimal reproductive rates, both the anatomical and physiological workings of the reproductive system must function properly. A basic understanding of the anatomical and physiological function of the female pig reproductive system can aid producers in anticipating and troubleshooting reproductive problems, and in facilitating decisions which impact performance of the breeding herd. This article introduces the reader to the anatomy and physiology of female reproduction and how this acts to enhance or inhibit performance.
Female Reproductive Tract
General Parts and Support
The female reproductive tract is composed of paired right and left ovaries, oviducts, and uterine horns. It contains only a single cervix, vagina and vulva (external genitalia). Collectively, these structures are supported by the broad ligament and hang loosely suspended below the rectum in the both pelvic canal and lower abdomen. The broad ligament is made of tough connective tissue, attaching near the point of the spine, and running continuously with the inner most layer of the abdominal cavity. Many of the blood vessels and nerves travel through this large piece tissue in order to supply the reproductive tract with blood, hormones and neural stimuli. In prepubertal gilts, the ligament is short, paper thin, and almost transparent. However, in late pregnancy it becomes very long as it stretches and thickens in order to support the increasing weight of the pregnant reproductive tract. The Ovary
The ovary of the pig is primarily important because it is the source for both reproductive hormones and eggs. The ovary is particularly responsive to important hormones that are released from other organs, especially those of the pituitary. The pituitary is located near the base of the brain and is the source of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). It is these two hormones which are responsible for initiating and stimulating the ovary to become active in order to begin reproduction. Note: PG600Ò is an approved drug for stimulating estrus in gilts and is very close in structure and function to FSH and LH. FSH causes many small follicles (< 3 mm in diameter) to grow into medium sized follicles (3 - 6.5 mm). These follicles appear as small, blister-like structures on the surface of the ovary (Figure 1). Each follicle contains an egg and produces considerable amounts of steroid hormones, most notably, estrogen. The other pituitary hormone, LH, is important for the continued growth of the medium sized follicles into large follicles, which are responsible for releasing the egg at estrus. As the follicles grow, the egg inside the follicle also begins to mature as estrogen levels inside the follicle become very high. This elevated follicle estrogen ultimately leads to increased estrogen levels in the blood. When the blood concentrations of estrogen become high enough, the female shows signs of estrus and eventually stands for the back pressure test in the presence of a bore. Ovulation
The occurrence of peak levels of estrogen in the blood, which originate from the large follicles on the ovary, is followed closely by a surge of LH into the blood at the time of estrus. Ovulation of the large follicles appears to occur at a specific time interval (~ 42 h) after this LH surge. The eggs from all of the large follicles from both ovaries ovulate in a relatively short period of time (~ 3 h). However, even though there is minimal variation in the time to ovulate all follicles within a female, the time of ovulation after onset of estrus is highly variable between females. Some sows are observed to ovulate as early as 24 h to as late as 60 h after onset of estrus. This variation in time of ovulation is greater than or equal to 24 h. Therefore this variation is...