There is a rhythm to nature. Season follows season, with each having a specific purpose to serve in the grand scheme of things. There is a time for planting, a time for growing, a time for harvesting, and a time for resting to prepare for the cycle to be repeated. It is much the same with reproduction in the horse.
Equine reproduction follows the same rhythm as plants, with one season blending into another; each has its own role to play in influencing when a mare will be ready to ovulate, conceive, and carry a foal to term. As mammals go, the horse is not the most efficient from a reproduction standpoint. The success rate varies, depending on circumstances, but it can be as low as 55%.
Why this is so cannot be easily answered. Some scientists will say that it is because many breeders do not avail themselves of the knowledge and technology that is available. Others will say it is because man has tinkered with nature in his attempt to get early foals. Still others will point out that the mare's reproductive system is highly complicated and must function like a finely tuned machine. And, like a finely tuned machine, if something untoward happens at any stage or at any location along the way, failure will be the result.
There likely is some truth to all of the above assertions. A discussion of the reproductive process can be long and complicated, so in this article, we will take a look at only one aspect that has a strong bearing on reproductive success: Ovulation.
The information that follows comes from a variety of sources, with emphasis on papers and reports from Colorado State University. We will also discuss information from Juan Samper, DVM, PhD, Dipl. ACT, an associate professor at Kansas State University, where he is in charge of the equine reproduction unit.
Ovulation is that point in the cycle when the unfertilized egg leaves the follicle and heads into the fallopian tube to, hopefully, meet up with sperm that is swimming toward it. Before we can understand the ovulation process, we must have some information about the organs involved--primarily the ovaries. Secondary to the ovaries in the ovulation scenario are the fallopian tubes, which provide a passageway for both the egg and the sperm.
First, the Ovaries
A mare's ovaries are unique among mammals, both in size and makeup. They are shaped like kidney beans and vary in size, depending on whether the mare is in estrus or is going through anestrus (the seasonal phase when her reproductive system has shut down).
During the non-breeding season, the mare's ovaries are hard to the touch and have shrunk to 2-3 centimeters in width and 2-4 centimeters in length. In contrast, during the breeding or sexually active season, the ovaries will increase in size to 6-8 centimeters in length and 3-4 centimeters in width. They also will be softer to the touch due to the development of fluid-filled follicles (vesicles that each contain a developing egg within a covering of cells).
The convex (curved outward) side of the ovary is suspended in the abdomen by the broad ligament. The blood vessels and nerves that serve the entire ovary pass through this region. The concave (curved inward) side of the ovary contains an area unique to mares--the ovulation fossa. It is only from this wedge-shaped area that an egg or ovum can be shed or ovulated.
The entire ovary--other than the ovulation fossa--is contained within a thick protective layer known as the tunica albuginea.
There are two layers in the internal structure of the ovary. The outermost layer is the medulla, which contains nerves and a blood supply. The inside layer is the cortex, and it is here where the eggs or ova are located. At birth, a filly already possesses (within the ovarian cortex) all of the egg cells that she will have in her lifetime.
Also located within the cortex are the follicles. Each egg or ovum is encased in a single layer of follicular epithelial cells; this is known as the primordial follicle. Only a small fraction of the primordial follicles present at birth will reach maturity and liberate (ovulate) their ova.
Following ovulation, the cortex will also contain a structure known as the corpus luteum. It forms from the tissues remaining after a follicle ruptures at ovulation. The corpus luteum is responsible for production and secretion of progesterone in the early stages of pregnancy.
Other important parts of the reproductive tract are the oviducts or fallopian tubes. Without them, ovulation would accomplish nothing. The job of the fallopian tubes or oviducts is to transport the ovulated egg toward the uterus, where it will implant and grow into a foal if fertilized.
The oviducts are tiny, highly coiled tubes. There are two of them, with each one connecting the tip of a uterine horn to an ovary. The portion of the tube connecting to the ovary is known as the infundibulum. It is enlarged and shaped a bit like a catcher's mitt, and it has finger-like projections known as fimbriae. Its unique design enables it to envelop or cradle the portion of the ovary from which the egg will emerge so that the egg can be captured and transported down the oviduct to the uterus.
Fertilization of the egg occurs in the oviduct in the area below the infundibulum called the ampulla. The portion of the oviduct where it narrows before joining the uterus is the isthmus.
Both oviducts are heavily lined with hairlike projections called cilia. They beat rhythmically to help transport the egg along the oviduct and to facilitate movement of sperm in the opposite direction, putting egg and sperm on nature's version of a collision course.
The spot where the oviduct connects with the uterus is known as the utero-tubular junction. If the egg is to arrive in the uterus, it must be fertilized within the oviduct because only a fertilized egg can pass through the junction and venture on into the mare's uterus. The equine egg is unique in that if it is fertilized, it produces the hormone prostaglandin E2. This hormone causes the oviduct to relax, allowing the passage of the fertilized egg. Eggs that are not fertilized or die within the first five days do not produce prostaglandin E2, and the oviduct remains snug around them. Eventually they degenerate.
Let the Rhythm of Nature Begin
The lead factor in beginning the reproductive season is light. If nature is allowed to function without interference, this will mean that as the days grow longer, the dormant reproductive system will begin to awaken, much like the Earth that absorbs sunlight and moisture to allow roots to push up grass shoots, and seeds to germinate.
In the case of wild horses, there is even more of an intertwining. As the sun awakens the mare's reproductive system, the green grass helps to supply additional nutrition that will further stimulate reproductive activity.
A mare's reproductive cycle is described as being seasonally polyestrous. This means that she has a reproductive season in which she cycles several times, and a non-reproductive season. Late fall and winter are known as the anestrus or non-reproductive season, while spring and fall normally constitute the reproductive season.
When a mare is in anestrus, she loses her sexual drive. An approach by a stallion will be met with pinned ears, squeals, snapping teeth, and probably kicking and striking. During the reproductive season, however, she will go through heats where she will accept the stallion's advances and will stand still while being mounted and bred.
She normally will only be receptive to the stallion's advances during several days of her estrous cycle during the sexually active season. These cycles will repeat themselves at 21- to 23-day intervals unless she becomes pregnant. Once she is pregnant, the mare again will reject the stallion's advances.
During each of the cycles, there is a five- to seven-day period when the mare is in estrus (heat) and is receptive to the stallion. It also is only during that period of time that she can become pregnant.
While there are two basic halves to the reproduction season, they actually can be split again, making four in all. For both basic seasons, there is a transitional stage, almost as though there is a warm-up and a cool-down period before and after the breeding season.
During the transitional phases, especially in the spring, the mares generally are erratic in their sexual behavior and even in their ability to become pregnant. For example, sometimes they will come into heat but will not ovulate.
This stage is like an orchestra tuning up for a concert. All of the instruments are present and are being warmed up, but they aren't being played in rhythm and unity. However, once the conductor raises the baton, all of the instruments are synchronized.
Hormonal Control of Estrus
As already indicated, the spring warm-up will begin with an increase in daylight or artificial light. As the mare's brain records the increased amount of light and higher temperatures, the hypothalamus gland located within tissues of the midbrain is stimulated. The hypothalamus gland signals the opening of the reproductive season by producing and secreting gonadotropin-releasing hormone (GnRH). When the proper amount of GnRH is secreted, the pituitary gland, located nearby at the base of the brain, is stimulated.
The pituitary gland plays a key role in the reproductive process, and is well designed for the task. It is a highly vascularized (with a lot of blood vessels) gland and, as such, can monitor levels of certain hormones in the bloodstream with a high degree of sensitivity. It also secretes two hormones--follicle stimulating hormone (FSH) and luteinizing hormone (LSH). The first stimulates follicular development within the ovaries, and the second induces the mature follicle to ovulate.
We mentioned earlier that a mare's normal cycle covers a period of 21 to 23 days. The concentration of FSH rises from Day 15 of that cycle onward. It will increase from levels of 4 nanograms per milliliter to as high as 9 nanograms per milliliter during estrus.
As many as 10 follicles within the ovary might be stimulated to grow and develop, but normally only one will mature to the point where it will be released.
The developing follicles also take on another role as they grow and mature. When the follicle or follicles reach 20-25 millimeters in diameter, estrogen is secreted. This hormone stimulates estrual activity so that the mare will come into heat and be receptive to the stallion. It also affects the cervix, allowing it to relax so that there is an open gateway into the uterus for the sperm; stimulates the smooth muscles in the mare's reproductive tract for increased contractions to transport sperm and ovum; signals the pituitary gland to shut down secretion of FSH; and stimulates the pituitary to release its second gonadotropic hormone--luteinizing hormone (LH).
The function of LH is to facilitate maturation of the growing, egg-bearing follicle, culminating in ovulation. When the egg reaches the proper stage of maturity, it is released by the follicle and begins its trip down the oviduct. Normally, this occurs late in the estrous cycle.
In the past, veterinarians were only able to palpate the ovary through the rectal wall in such a manner that they could feel the developing follicle. They then had to guess when it might reach the proper stage for ovulation.
Today, the ultrasound endoscope has removed much of the guesswork. Not only is the veterinarian able to visualize the ovary via rectal ultrasound exam, he or she can also measure it and normally make a more accurate prediction as to when the mare will ovulate. The owner can then schedule a breeding date accordingly.
Samper presented a paper at the 1997 meeting of the American Association of Equine Practitioners (AAEP) wherein he described an added dimension or tool in determining when ovulation is imminent. Today, he says, he is more convinced than ever that the approach is a reliable indicator for time of ovulation.
The process involves determining and scoring the amount of edema (fluid swelling) present in the uterus. "The uterus, under the influence of estrogen," explains Samper, "will display a characteristic pattern of edema that could be described as a cartwheel pattern. Once the mare has ovulated and the reproductive tract begins to be under the influence of progesterone, the endometrial edema is no longer present under normal circumstances."
Samper uses a subjective scoring system (called SEE, which is an acronym for Score of Endometrial Edema) that ranges from 0 to 5 to determine the amount of endometrial edema present. A score of 0 was given to mares that were in diestrus and had no uterine edema, and a score of 5 was assigned to mares with maximal uterine edema. The score of 5 would indicate that ovulation was imminent. The scores in between indicate lesser amounts of edema.
When the SEE score nears or reaches 5, Samper recommends administering hCG (human chorionic gonadotropin) to help facilitate ovulation during a predictable time frame.
The approach is highly successful, Samper says, because, "In a normal mare, the pattern of uterine edema is very consistent. It is a reliable method for determining when ovulation will occur."
There is another plus. "Abnormalities in the appearance and disappearance of uterine edema will help veterinarians predict which mares should have a uterine culture (to identify an infection) in addition to those that might need post-breeding therapies," he adds.
The amount of uterine edema present, Samper says, can be determined through rectal palpation and ultrasound.
Immediately following ovulation as the estrogen level falls, the now-empty follicular cavity from which the egg ovulated accumulates coagulated blood and forms the corpus hemorrhagicum.
Luteal cells within the corpus hemorrhagicum grow toward the interior, eventually replacing the clotted blood with a solid core of luteal cells. When this is achieved, the structure is called the corpus luteum.
The luteal cells within the corpus hemorrhagicum and corpus luteum secrete the hormone progesterone. Under this scenario, the progesterone level rises as the estrogen level falls. Progesterone is the key hormone in maintaining a pregnancy. Its first job is to shut down the actively contracting reproductive tract and to tighten and close the relaxed and open cervix.
At the same time, progesterone inhibits the secretion of FSH and LH from the pituitary gland. When this has been accomplished, the mare goes into a state of diestrus--out of heat. No longer is she receptive to the stallion's advances.
What occurs next depends on whether a pregnancy occurred. If it did not, the uterus will remain under the influence of progesterone for 12 to 14 days, then changes will occur and the entire process will start over again. If the mare has become pregnant, the presence of the conceptus will extend the life of the corpus luteum (and its product of progesterone) for 35-90 days, until the conceptus can produce progesterone on its own.
Science has removed much of the guesswork on the timing of ovulation, but at the same time, man's quest for foals born early in the year when mares are not usually cycling has only allowed for science to hold its own in the battle for higher reproduction rates.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals