Equine Reproduction--The Essential Elements

Equine Reproduction--The Essential Elements

Man has decided that, for most of the athletic breeds, the goal is to produce foals as soon after Jan. 1 as possible. This battle with the innate traits of the species results in much of the reproductive inefficiency for which the horse is known.

Photo: Anne M. Eberhardt/The Horse

Understanding some of the evolutionary history of horses is helpful when pondering their reproductive complexities. After evolving into a grazing animal with great agility and speed (about a million years ago), the modern horse migrated to various parts of the Earth in response to changing physical and weather conditions. After near-extinction 10,000 years ago, the species came back in Asia and gradually repopulated parts of the world as man domesticated and transported the animals. Their return to North America was only 500 years ago, when the Spanish explorers re-introduced horses.

In each diverse area of the world, reproductive function is molded by the environment. Since the gestation period of equids is nearly one year in most cases, adaptation to a breeding season that corresponded to the best weather and grass seasons was important to survival. This seasonality is controlled by several factors, with light being the primary agent. The interaction between lengthening days and melatonin, secreted by the pineal gland, is the control mechanism. Melatonin, which has an inhibitory effect on reproductive function, is reduced as day length increases, releasing the hormonal control of the mare's cycle.

Thus, the horses of the desert, or other equatorial regions with earlier increases in day length, produce foals throughout a broad season, and the ponies which originate in far northern areas have the shortest breeding and foaling seasons. Breeds that were developed in Britain, northern Europe, and North America are at peak reproductive efficiency between May and August in the northern hemisphere. Arabians, Paso Finos, and breeds of more southerly origin might reproduce all year.

Unfortunately, man has decided what is best, and for most of the athletic breeds, the goal is to produce foals as soon after Jan. 1 as possible. This constant battle with the innate traits of the species results in much of the reproductive inefficiency for which the horse is known. Even with the assistance of artificial lighting to advance the season, and with hormonal systems of manipulating the cycle, we do not approach the potential results attained by those animals which are driven by the combination of long days and good, natural feed.

The reason for the improved reproductive rate in summer is related to the length of estrus (heat period) of mares and to sperm cell production by stallions. Estrus is prolonged in early spring as the reproductive system "tunes up." By May and June, estrus lasts only a few days. The ova (eggs) produced by mares live only for a few hours after they are ovulated. Sperm cells, on the other hand, typically survive two or more days in the reproductive tract after breeding. Therefore, one breeding, before ovulation, usually is sufficient for conception in the summer. Sperm production also is more efficient in long days, with stallions typically doubling sperm numbers from winter to summer.

The Mare

Let's examine the estrous cycle of a normal mare which will be bred to a fertile stallion and will conceive. That will help explain the intricate processes involved in equine reproduction. The goal of this cycle is to deliver a fertile egg into the reproductive tract, where it can be fertilized. Eggs are produced by the ovaries of the mare in response to hormonal control. Briefly, a hormone which will stimulate the pituitary gland is released from a portion of the brain called the hypothalamus. This compound is called GnRH, or gonadotropin releasing hormone. GnRH stimulates the pituitary gland to produce two hormones, FSH and LH, which are necessary for development and maturation of the follicle, a structure on the ovary that produces the egg. As the follicle progresses, it releases estrogen, the hormone which results in the mare's becoming sexually receptive. The mare now is in estrus, or in heat, and stands when presented with a stallion. (Most mares are difficult to detect in heat without using a stallion.)

When the follicle is fully matured, it ruptures (ovulates), releasing the egg from the ovary close to the opening of the fallopian tube (oviduct). For an instant the egg is free in the abdominal cavity, but the specialized structures of the oviduct quickly capture it and start it moving down the tube. Assuming the mare was bred prior to ovulation, fertile sperm cells are waiting for the egg in the upper portion of the oviduct, where fertilization takes place. (More about the travels of sperm cells in a moment.) The fertilized egg will spend about a week in the oviduct, migrating down the tube before it enters the uterus.

Shortly after ovulation, a key event takes place in the ovary at the site of the recently ovulated follicle. Cells from the follicle lining organize themselves into a glandular structure capable of producing the hormone progesterone, which is needed to establish and maintain pregnancy until the placenta is fully functional at about 150 days of gestation. The gland is called the corpus luteum (CL), and its presence is needed only if pregnancy is established. Progesterone immediately begins to change the behavior of the mare, as she becomes unreceptive to the stallion shortly after ovulation takes place. But progesterone also plays a major role in control of the secretions of nutrients by the cells lining the uterus, critical for the support of the embryo.

If pregnancy is not established by about 14-15 days after ovulation, the uterus is signaled to start a new cycle by the absence of an embryo. It then will produce a compound called a prostaglandin, which will terminate the life of the CL, dropping progesterone levels. This takes the brakes off of the cycle and allows a new sequence of GnRh, FSH, and LH to occur. The mare comes back into heat and repeats the whole cycle. On average, mares ovulate about every 21 days during the peak of their breeding season.

(For a more detailed consideration of this subject, see "Regulating Estrus in Mares" on TheHorse.com.)

The Stallion

Development of the sperm cells by the stallion actually is quite similar to the development process in the mare. For example, the hormone control is essentially the same. GnRh is the driving force in sperm cell production, stimulating the pituitary gland to produce the hormones FSH and LH. This sequence is necessary for the primary germ cells (spermatogonia) to begin the process of cell division and maturation that results in a sperm capable of fertilization. The process takes place in the tubules of the testicles, and the entire development of each sperm cell takes about 60 days.

In contrast to the production of one or two ova per cycle in mares, sperm cells are produced in great numbers, and the process is continuous all year. The effect of season is an increase in the number of sperm produced in long daylight seasons rather than to shut down during short day seasons.

When each sperm cell approaches maturity it is released from the tubule and migrates through a specialized portion of the testicle called the epididymis. There, the final maturation process takes place that enables the sperm to penetrate an egg and fertilize it. These mature spermatozoa are stored until the stallion ejaculates, when they are transported through the urethra to the mare's uterus. Along the way, there is a contribution of specialized fluids from accessory glands that contribute the liquid portions of the final product, which is called semen.

Billions of sperm cells are produced daily by fertile stallions. A typical horse of a light-horse breed releases five to 10 billion cells at each ejaculation. Of these, usually 50% or more are motile, or moving in a progressive manner, and a large percentage have normal structure, or morphology. When a stallion breeds a mare, the semen is deposited directly into the uterus. The large number of sperm quickly is reduced as they migrate up the horns of the uterus to the opening of the oviduct. As few as 5,000 to 10,000 cells actually enter the oviduct to participate in the fertilization process. Most of these are stored at one of two storage sites in the oviduct, as they wait to fertilize the ova that will migrate down toward them.

The minimal number of normal, motile sperm cells deposited in the uterus that are needed to achieve fertilization has been estimated at 100 to 500 million cells. The lower figure probably is about right for artificial insemination, and the higher number for natural service. It is interesting to note that the many billions of sperm that do not migrate into the oviduct are not totally wasted. They become an important signal to the uterus to begin contracting and to release white blood cells, both important components of the "uterine defense process" that helps clean up the uterus in preparation for the arrival of the fertilized egg or embryo.

For the majority of stallions with adequate sperm numbers, the limiting factor affecting the number of mares they can breed is their libido, or their willingness to breed mares. This is another stallion factor that is affected by season. Long days equal more sexual desire and more sperm cells produced.

The Embryo

So, we have delivered a fertile ovum into the oviduct of a mare which has been covered by a fertile stallion, so that healthy sperm are available. The union of these two cells--sperm and egg--will result in a combining of the genetic material from each parent into an embryo that eventually will develop into a fetus, then a foal. Cell division commences immediately after the fertilization process, and the embryo begins its descent through the oviduct to the uterus. It is protected from the environment of the uterus during this trip because the uterus still is in the "cleaning up" process for at least a few days. The embryo, which is now a ball of cells (blastocyst) measuring a little less than one millimeter, drops into the uterine cavity in an average time of six days after ovulation.

For the next week, as the embryo continues to develop, it migrates through the uterus, contacting the lining as a signal to the mare that she is pregnant. By about 15-16 days, the migrating stops and the embryo is fixed in one place, usually at the base of one uterine horn. If the signal has been received and interpreted, the uterus will not release prostaglandin, so that progesterone production will be continued by the CL, and the mare will not return to estrus.

Modern Medicine

Using the modern ultrasound equipment available today, veterinarians can identify the equine embryo at 12-14 days, shortly before it is "fixed" in the uterus. Identification of twin pregnancies by this time is considered important if reduction of one embryo is the goal.

A.C. Asbury, DVM

Unlike many mammals, the embryo of the horse will not have nutritional support from a placenta until quite late. The placenta is developing in the third month of gestation, and probably functional somewhat after 100 days. In the meantime, the embryo is nourished by the yolk sac, which it has developed itself, or by secretions from the uterine glands. These glands are driven to produce this nourishment by progesterone, the hormone produced by the CL. In order to beef up this production, mares develop extra CL tissue at about 42 days of gestation, which will boost progesterone through the critical period, up to at least 120 days of gestation. The mechanism thought to be critical to the formation of these new glands is a hormone produced by the endometrial cups called eCG, or equine chorionic gonadotrophin. This hormone can be detected by testing the blood after about 40 days, and provides the basis for a pregnancy test, similar to that used in women.

The Fetus

After all of the organ systems of our conceptus have been differentiated, we call the future foal a fetus instead of an embryo. This term is applied after about 40 days of pregnancy. From here to term, at 305-365 days, it's basically just growth that's involved. This growth is very rapid, from a few ounces in body weight to around 100 pounds at term in many breeds.

An excellent description of the developmental processes of foals can be found in the article Fetal Development And Foal Growth on TheHorse.com.


Now that you have some basic facts concerning reproduction in the horse, let's be sure they can all be put together to give you the most beneficial results.

Fact: Breeding efficiency of horses peaks during the longest days of the year.

Response: If you must breed earlier in the spring, use artificial lighting to move the mare's season forward. The simplest method is to provide light for 16 hours a day, adding to daylight at the end of the day. In other words, have the mare under light by sundown and leave her there (or turn the lights off) after the 16th hour. A 200 watt bulb in a 12 x 12 stall is sufficient. Start 60 days before you want the mare to cycle. Discuss this with your veterinarian to be sure you've got it right!

Fact: It is easier to detect signs of sexual receptivity (heat) when mares are introduced to a stallion.

Response: Teasing mares with a stallion and recording the information on a regular basis are probably the most important things you can do to achieve high reproductive efficiency. Find a way to tease your mares, whether with a pony stallion, the neighbor's horse, or whatever. While it is possible to get the job done without teasing, it requires extensive, repeated examinations...and then you get fooled fairly regularly.

Fact: The ovum lives but a few hours after ovulation, while sperm cells typically survive 48 or more hours in the mare's reproductive tract.

Response: Breed mares before they ovulate. Breed again if ovulation does not occur by 48 hours. This requires assistance from your veterinarian in predicting ovulation time and confirming ovulation occurrence. Rectal palpation of the ovaries and sometimes ultrasound examination are necessary for this response.

Alternative: Turn the stallion and mares out together and let them decide.

Fact: Twinning is highly undesirable in horses. The great majority of twins are aborted before they are born, or fail to develop into viable foals. Modern ultrasound technology and skillful manual dexterity by your veterinarian allow early detection of twins and reduction to a single pregnancy. Emphasis here is on the word early. Results of reduction are much better before 20 days, and the process is much easier on the mare.

About the Author

A.C. Asbury, DVM

A. C. (Woody) Asbury received his DVM from Michigan State University in 1956, then spent 21 years in California in breeding farm practice and at UC Davis. He joined the faculty at the University of Florida in 1977 and was involved in teaching, research, and administration until 1996. An Emeritus Professor at Florida, he lives in Kentucky, where he and his wife are developing a small farm.

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