 |
 |
| Basics of Reproduction by W.E. Beal, Virginia Tech, and J.M. DeJarnette, Select Sires Inc. CLICK HERE TO LISTEN TO HIS PRESENTATION You will need Windows Media Player to listen in.
Female reproductive tract  First, consider the parts that make up the female reproductive system in cattle. There are two ovaries, two oviducts, two uterine horns, a uterine body, the cervix, the vagina and the vulva. The bladder lies below the reproductive tract and is connected at the urethral opening on the vaginal floor. The rectum is located above the reproductive system. The vulva is the opening to the reproductive system. Its three functions are to pass urine, to serve as the opening for mating, and to serve as the birth canal. Included in this structure are the lips, vestibule and clitoris. The vulval lips are located at the opening and appear wrinkled and dry when the cow is not in heat. One of the secondary signs of heat is swelling of the vulva, along with a moist, red appearance.The vagina, about 4-5 inches (in.) in length, extends from the urethral opening to the cervix. This is the site where semen is deposited during natural mating. Cells in the vagina and cervix secrete mucus, which lubricates the tract during insemination.The cervix is a thick-walled organ forming a connection between the vagina and uterus. The cervical opening protrudes back into the vagina. The cervix itself is made of very dense connective tissue. Thick folds and rings in the center of the cervix overlap, making an effective barrier between the uterus and vagina. The cervix opens into the body of the uterus. About an inch long, the body of the uterus serves as a connection between the two uterine horns. This is where semen is usually deposited during artificial insemination (AI). From this point, the reproductive tract separates, and all further structures described come in pairs. The two uterine horns consist of layers of muscle and a heavy network of blood vessels. When a cow is bred — either naturally or by AI — the uterine muscles, under the influence of the hormone oxytocin, rhythmically contract. Further contraction by muscles of the oviduct help move semen to the middle of the oviduct, the site of fertilization.Oviducts, as their name implies, carry ova, the cow’s eggs. Ova are caught by the large open end of the oviduct, which surrounds the ovary. This funnellike structure, called the infundibulum, keeps the ova from falling into the body cavity. Hairlike structures carry the ova down the oviduct to the site of fertilization. After fertilization, the embryo travels down the oviduct and arrives in the uterus within 4-5 days.The ovaries are the primary organs in a cow’s reproductive tract. They have two functions: to produce ova (eggs) and to produce hormones, such as estrogen and progesterone, throughout the different stages of the estrous cycle.The estrous cycle Over a period of time, many changes take place in the reproductive system. These changes in nonpregnant, normal cows repeat every 21 days. This regular repetition
 Consider the estrous cycle as beginning when the cow is in heat on Day Zero. Looking at the reproductive tract, several things are happening. One ovary has a large follicle with a blisterlike appearance. This follicle, filled with a fluid, has a mature egg ready to be released. The follicle is also producing the hormone estrogen. Estrogen, transported in the blood to all parts of the cow’s body, causes other organs to react in a number of ways. It makes the uterus sensitive to stimulation and aids in the transport of semen at the time of insemination. It causes the cervix to secrete viscous mucus that flows and lubricates the vagina. Estrogen is also responsible for several signs of heat in a cow: a red, swollen vagina; allowing other cows to mount her; going off feed; bellowing; and holding her ears erect.On Day 1, the follicle breaks, releasing the egg into the oviduct. In the oviduct, the egg awaits sperm for fertilization. By the time the follicle has broken, most cows will have quit producing estrogen. As a result, the cow no longer displays the familiar signs of heat.
New types of cells, luteal cells, grow in the void on the ovary where the follicle was located. Quite rapidly over the next five to six days, the corpus luteum (CL) grows.The CL produces another hormone, progesterone. Progesterone readies the uterus to accept a fertilized egg and keeps the cow from coming in heat. Under the influence of progesterone, the uterus produces a nourishing substance for the embryo. At the same time, a thick plug forms in the cervix, preventing any bacteria or viruses from entering the uterus.Progesterone also blocks the release of follicle stimulating hormone (FSH) from the pituitary gland. This keeps a pregnant cow from coming back in heat every three weeks, which would make life difficult for a new embryo. On the other hand, if the cow is not pregnant, we do want her to come in heat again. Sixteen or 17 days after the cow was in heat, if the uterus does not detect a growing embryo, it begins to produce another hormone, called prostaglandin. Prostaglandin begins to destroy the CL.With the CL gone, no more progesterone is produced, and the pituitary gland begins to secrete FSH, which goes to the ovary.The FSH initiates the development of a new follicle. The new follicle grows, producing large amounts of estrogen and bringing the cow back into heat. A full cycle is now completed. The average total time is about 21 days, with a normal range of 18 to 24 days. Embryo and fetal development Compare the events during the estrous cycle with those that occur if the cow is inseminated and becomes pregnant. For the first four to five days, the embryo would move in the oviduct toward the uterus. Once it enters the uterus, the embryo would be bathed in uterine fluids and continue growing, while floating free in the uterus for about 30 days. If the cow were pregnant, the events of the estrous cycle would be the same as those of the nonpregnant animal until Day 16 or Day 17. An embryo present in the uterus at that time will produce a substance that interferes with the ability of prostaglandin to destroy the CL.Thus, the CL is not destroyed and remains on the ovary, producing large amounts of progesterone to support embryo development and to prevent the cow from coming back into heat. This CL will remain on the ovary until the cow is at or near calving.Several membranes — including the amnion, the chorion and allantois — are produced by the new embryo, and they become attached to the uterus at several points. These points of attachment are called placentomes. They allow the calf to get nutrients from the mother and dispose of wastes by arteries and veins going in and out through the umbilical cord.At calving, the muscles in the uterus begin to contract and eventually expel the calf and membranes through a dilated cervix and vagina. Several hormones produced by the mother, including progesterone, estrogen, prolactin and corticoids; the fetus; and the placenta interact to bring about this event.Calving in a clean environment and proper treatment of the cow after a difficult calving will help prevent reproductive problems. Becoming familiar with the reproductive anatomy and physiology of a cow helps all producers do a better job in the reproductive management of their herd.
home | conference schedule | tour schedule | conference speakers | hotel info |
