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Understanding Females

If you're frustrated with results from your artificial insemination (AI) program, don't give up. Improvements can often take place easily, especially if you bolster your basic understanding of three things, says W.E. Beal of Virginia Tech University and Mel DeJarnette of Select Sires.

Knowledge of these three things — the cow's reproductive tract and how it works, the estrous cycle and what it means, and embryonic/fetal development after fertilization occurs — should result in more AI-sired calves on the ground next year and vast improvements in the genetic quality of your cowherd.

“Perhaps the most important of these is understanding the events, timing and hormonal controls of the estrous cycle,” says Beal, a Virginia Tech animal scientist. “Understanding the estrous cycle is fundamental to explaining the things that go wrong if a female fails to become pregnant or experiences embryonic mortality.”

Reproductive Tract

Let's start with the female reproductive tract. Basically, it consists of two ovaries, two oviducts, two uterine horns, a uterine body, the cervix, vagina and vulva. The bladder lies below the reproductive tract and is connected at the urethral opening on the vaginal wall. The rectum is located above the reproductive system.

The vulva is the opening to the reproductive system. Its functions are the passage of urine, the opening for mating and as the birth canal. Included in the vulva's structure are the lips, vestibule and clitoris. The vulva lips are located at the opening and appear wrinkled and dry when the cow is not in heat. A secondary sign of heat, however, is when the vulva swells and appears moist and red.

The vagina, which is 4-5 in. long, is located from the urethral opening to the cervix. This is where semen is deposited during natural mating. Cells in the vagina and cervix secrete mucus, which lubricates the tract during insemination.

“The cervix is the thick-walled organ that connects the vagina and the uterus,” explains Beal. “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 it an effective barrier between the uterus and the 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 AI.

The two uterine horns consist of layers of muscle and a heavy network of blood vessels. When a cow is bred either naturally or through AI, the uterine muscles, under the influence of the hormone oxytocin, rhythmically contract. This continuous contraction helps move the semen to the site of fertilization in the middle of the oviduct.

Oviducts carry ova, the cow's eggs. The ova are caught by the large open end of the oviduct, which surrounds the ovary. This funnel-like structure, called the infundibulum, keeps the ova from falling into the body cavity. Hair-like 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 four to five days.

“The ovaries are the primary organs in the cow's reproductive tract,” says DeJarnette. “They have two functions: production of ova and production of 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 a cow's reproductive system. These changes in non-pregnant, normal cows repeat about every 21 days. This regular repetition is called the estrous cycle.

“Consider the estrous cycle as beginning when the cow is in heat on day zero,” explains Beal. “Looking at the reproductive tract, several things are happening. One ovary has a large follicle with a blister-like appearance. This fluid-filled follicle has a mature egg ready to be released.”

The follicle is also producing the hormone estrogen. Estrogen, which is transported in the blood to all parts of the cow's body, causes other organs to react in numerous ways:

  • Makes the uterus sensitive at the time of insemination.

  • Causes the cervix to secrete a viscous mucus that flows and lubricates the vagina.

  • Results in a red, swollen vagina.

  • Disposes the cow for mounting by other cows.

  • Often causes the cow to go off feed, to bellow or hold 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, this cell growth, called the corpus luteum (or CL), continues to grow.

The CL is important because it produces another hormone called progesterone, which makes the uterus ready to accept a fertilized egg and keeps the cow from coming in heat.

Under the influence of progesterone, the uterus produces a nourishing substance on which the embryo grows. At the same time, a thick plug forms in the cervix, preventing any bacteria or viruses from entering the uterus.

“Progesterone also blocks the follicle stimulating hormone (FSH) from the pituitary gland,” adds DeJarnette. “This keeps a pregnant cow from coming back in heat every three or four weeks, which would make life very difficult for the new embryo. On the other hand, if the cow is not pregnant, we do want her to come in heat again.”

If the cow doesn't breed, 16 or 17 days after heat, the uterus 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, which grows and produces large amounts of estrogen. This ultimately brings the cow back into heat. The full cycle is now complete. The average total time is about 21 days with a normal range of about 18-24 days.

Embryonic, Fetal Development

If the cow becomes pregnant, the events of the estrous cycle are the same as those of the non-pregnant animal until Day 16 or 17. That's when an embryo present in the uterus begins to produce a substance that interferes with the ability of prostaglandin to destroy the CL.

Thus the CL remains intact and on the ovary producing large amounts of progesterone to support the developing embryo and prevent the cow from coming back in heat. The CL remains on the ovary until the cow is at or near calving.

During the first four or five days of pregnancy, the embryo moves in the oviduct toward the uterus. Once it enters the uterus, the embryo is bathed in uterine fluids and continues growing, while flowing freely there for about 30 days.

Several membranes, including the amnion, chorion and allantois, are produced by the new embryo. These eventually attach to the uterus at several points, which are called placentomes. The placentomes allow the developing calf to get nutrients from its mother and dispose of wastes via 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, including progesterone, estrogen, prolactin and corticoids produced by the mother, the fetus and the placenta interact to bring about this event.

“Making sure you calve in a clean environment and provide proper treatment for the cow after a difficult calving will prevent reproductive problems in the future,” says Beal.

Adds DeJarnette: “It's the little things that add up. When a piece of equipment or machinery is broken, it's impossible to fix it unless you thoroughly understand the parts and how they normally work. The same is true with your cows and your reproductive management program.

“The more you understand about the cow, the more your management will cater to her needs. This will improve your AI results and the quality of your cowherd at the same time,” he says.

This article is provided by the National Association of Animal Breeders (NAAB). Find out more about NAAB by checking their Web site at or call 573/445-4406.