Short Cycling Mares

Nature has done a commendable job in developing the equine reproductive system, but it did not take into consideration man's special needs for the production of horses at certain times or under specific circumstances. The first man-made rule for some breeds involved birth dates. People decided for some registries that every equine was a year older on Jan. 1 following its birth year. This means it is important for foalings to occur early in the year to allow the youngster as much time as possible to grow and develop that first year. Soon, the demand for large, well-developed, robust yearlings at public sales gave added impetus to that desire.

That wasn't the end of the story. Along came artificial insemination. A single stallion could breed many more mares with artificial insemination than with natural cover. Then, even that procedure called for refinement. Many mare owners wanted their mares bred at the same time. Even by splitting ejaculates, breeding farms sometimes were hard-pressed to satisfy the needs of their clients.

Science had an answer. Manipulate the estrous cycles of mares so that the breeding load could be spread out.

Embryo transfer also brought with it some special needs. The estrous cycle of the recipient mare had to be synchronized with that of the donor.

Today, veterinarians routinely short-cycle or manipulate the reproductive cycle in mares to cause them to come into estrus at a designated time rather than when nature would dictate. The simplest of the procedures is short-cycling. The injection of a particular hormone at the propitious time can cut a mare's normal estrous cycle in half (or more). However, that is only one part of the overall picture. Much more is involved, and we will take an overall view in this article.

Hormones To The Rescue

The prime tools involved in manipulating reproductive cycles are two naturally occurring hormones--prostaglandin and progesterone--or their synthetic counterparts. Before discussing how they are used by man, let us take a moment to review how they are used by the mare when Nature's plan is followed without manipulation.

The mare's reproductive activity is seasonally polyestrous. This means she has a reproductive season and a non-reproductive season. Both are controlled by light. The non-reproductive season, known as anestrus, comes in the fall and winter when there is little natural light. The reproductive season begins in the spring (when there is increasing light) and continues through the summer.

Between those two seasons are two other cycles, known as transitional stages. One occurs just before the mare becomes reproductively active in the spring, and the other occurs just prior to anestrus in the winter. During these periods, mares generally are erratic in their cyclic and sexual behavior.

As already mentioned, light is the key element in bringing the mare into a state of estrus. It jump-starts the reproductive system by stimulating the hypothalamus gland located within tissues at mid-brain. The hypothalamus produces gonadotropic-releasing hormone (GnRH). When enough GnRH is produced, the pituitary gland at the base of the brain is stimulated. The pituitary then secretes two hormones that act on the ovaries. The first hormone is known as follicle stimulating hormone (FSH). It moves via the bloodstream to the ovaries, where it stimulates the production of one or more follicles.

When the follicles reach 20 to 25 millimeters in diameter, they secrete estrogen, which stimulates sexual activity in the mare. It prepares the reproductive tract and cervix for the arrival of sperm and causes the mare to be receptive to the stallion's sexual advances.

Estrogen also stimulates the release of a second gonadotropic hormone. It is luteinizing hormone (LH). This hormone facilitates maturation and ovulation of the growing, egg-bearing follicle. Ovulation occurs when the mature egg leaves the follicle and begins its trip through the oviduct, generally late in estrus.

Following ovulation, the estrogen level falls, and the remains of the ovulated follicle are converted to form a corpus luteum (CL) or yellow body. Luteal cells secrete the hormone progesterone. Its task is shutting down the secretion of the estrous-stimulating hormones, thus setting the stage for maintenance of a pregnancy.

As progesterone carries out its tasks, the cervix closes, contractions along the reproductive tract cease, and the mare loses all interest in the stallion's advances.

What happens next is dependent on whether the mare became pregnant. If she did, progesterone, under Nature's plan, will continue to function in its role of maintaining a pregnancy by suppressing other hormonal activity. If pregnancy did not occur, the uterus remains under the influence of progesterone for 12 to 14 days. After that time, changes will take place to prepare the way for another try at a pregnancy.

If an embryo is not present in the uterus at the end of 12 to 14 days, the uterine endometrium secretes the hormone prostaglandin. The prostaglandin destroys the corpus luteum that is producing progesterone. With progesterone production shut down, the level of FSH rises to the point where the whole cycle starts over.

Reproductive Pioneers

Through the years, scientists have zeroed in on progesterone and prostaglandin as key components involved in manipulating a mare's reproductive cycles to maximize a stallion's breeding capability and to set the stage for embryo transfer. Cooled shipped semen has added another dimension to the situation. In some breeds or disciplines--cutting horses, for example--major stallion owners cater to mares which are being bred on-premise. In a number of cases, they will not guarantee shipment of cooled semen on the exact date requested by a mare owner. This means that the mare owner might have to stand ready to short-cycle a mare so that she can be bred when shipped cooled semen is available. Frozen semen is not an option at the moment in the American Quarter Horse Association registry.

A good deal of pioneering work in the use of progesterone and prostaglandin in controlling the estrous cycle in mares has been carried out at Colorado State University under the direction of E.L. Squires, MS, PhD. Much research has been conducted at other institutes, as well.

One of the first research goals was to smooth out the erratic transitional period between anestrus and estrus. During that time, as mentioned, the mare often does not ovulate. If she does, it is erratic.

Researchers quickly learned that progesterone, administered either intramuscularly while suspended in oil or in the feed via synthetic altrenogest (Regumate), could provide a major assist, but that it was not magic. They learned that if they mimicked Nature by providing artificial light to jump-start the hypothalamus gland, the administration of progesterone was more effective.

The CSU artificial lighting program makes use of a 200-watt bulb in an ordinary box stall. The bulb is used to provide light to the mare's eye for 16 of the 24 hours each day for 60 days prior to treatment with progesterone. Providing light does not elicit an instant response in the mare. Generally speaking, a behavioral response can be seen 30 to 60 days after the program is initiated, with the first ovulation often occurring 60 to 90 days after the lighting program begins.

In one study, mares were exposed to a 16-hour photoperiod for 60 days prior to treatment with altrenogest for 12 days. The average interval to estrus after the end of the progesterone treatment was 3.4 days, and the majority of treated mares ovulated within 12 days.

In a report on the study, Squires had this to say, "The combination of artificial photoperiod and progestin (progesterone) treatment appeared to be an effective regimen for hastening the onset of the breeding season. If mares are not first primed with artificial light, then progestin treatment should be delayed until multiple follicles (20 millimeters or more in diameter) are present on the ovaries. Based on the results of a large clinical trial, the best candidates for a 14-day treatment with altrenogest were mares in extended estrus (10 days or longer) whose ovaries contained several follicles 20 millimeters or more in diameter."

Things smooth out a bit, and cycles become easier to manipulate once mares have moved through the transitional period.

"Once mares have established a normal cycle, control of estrus with progestins is less variable than in transitional mares," noted Squires.

There are two basic approaches to manipulate a mare's cycle. One involves progesterone alone and the other involves progesterone and prostaglandin.

"Several studies in our laboratory," Squires said, "evaluated the use of altrenogest for controlling estrus in cycling mares. Treatment was initiated at various stages of estrus and diestrus and continued for 12 days. Mares in estrus at the onset of treatment ceased to display estrus within two days; however, seven to 10 mares in estrus at the onset of treatment ovulated during treatment. Others (researchers) have also reported that ovulation occurs during progesterone treatment. Thus, if progestin is used alone, treatment should be for 14 to 15 days in order to allow time for corpus luteum (CL) regression.

"Since most breeders are interested in shorter treatment regimens for estrous synchronization, prostaglandin often is given on the last day of progestin treatment. This allows the progestin treatment regimen to be shortened to seven to 10 days."

However, true short-cycling can be achieved by administering prostaglandin alone. Timing is the key. If prostaglandin is administered when the mare is in a state of diestrus--out of heat--it has the same effect as the prostaglandin produced by the endometrium of the uterus when there is no pregnancy. The corpus luteum is immediately rendered incapable of producing progesterone. When this occurs, other hormonal levels instantly rise and bring the mare into estrus within a few days.

Using prostaglandin alone when the mare is in a state of estrus would normally be of no benefit. Using prostaglandin alone to manipulate mares that are in the transitional stage is basically ineffective.

An effective use of short-cycling with prostaglandin might be in the following situation: A mare has just gone out of a heat, and the stallion station reports that shipped cooled semen will be available several days hence. The mare could be short-cycled with prostaglandin in an effort to hasten the onset of the next period of estrus so that it would coincide with the shipment of semen. It should be noted that the corpus leteum will not respond to prostaglandin until it is "mature," or five to six days after ovulation has occurred. Prostaglandin treatment prior to five days will be unsuccessful. Therefore, in situations where the return to estrus is required in the shortest possible time, it is helpful to have the time of ovulation determined by either palpation or ultrasound examination.

Another example might involve the stallion which has a number of mares booked and several come into heat at the same time--more than he can cover effectively. By short-cycling one or two, all could be bred within a relatively short period of time rather than missing an entire heat cycle.

Prostaglandin alone (if administered within the correct time), progesterone alone (again, given within the correct time), or a combination of progesterone and prostaglandin can serve to bring a mare into heat at a predictable time. However, the two hormones have no real effect on exactly when the mare will ovulate. This has been a chink in the manipulation armor for some time.

Enter two other hormonal weapons that can be used in conjunction with progesterone and prostaglandin, or on their own, to hasten the onset of ovulation--human chorionic gonadotropin (hCG) and gonadotropin releasing hormone (GnRH).

Human Chorionic Gonadotropin

Research revealed that the two hormones could hasten the time to ovulation once a follicle was present and had reached a particular stage of development and, thus, help pinpoint the optimum time for breeding the mare.

Researchers found that a single intramuscular or intravenous injection (2,500 IU) of human chorionic gonadotropin at the appropriate time--normally when a 35 to 40 millimeter follicle is detected--during estrus generally results in ovulation within 24 to 48 hours of administration in normally cycling mares.

Another study found that hCG could hasten time to ovulation in mares which were in the transitional period. In this particular study at Colorado State, 38 light horse mares were used. When follicles of 20 millimeters in diameter were detected, the mares were assigned randomly to either a control group or a treatment group. When follicles reached 40 millimeters in diameter and the mares had demonstrated estrous activity for three days, the treatment group was injected intravenously with hCG. The majority of the treated transitional mares ovulated in one to six days after treatment. Nearly all of the mares in the control group ovulated later and displayed estrous behavior for 10 days or more before ovulating.

The only adverse effect reported to date on the use of hCG was proffered during the Society for Theriogenology session in Baltimore in 1998 by Nigel Perkins, a researcher from New Zealand. He said a study there revealed that use of hCG increased the incidence of twins.

Gonadotropin-Releasing Hormone

In a study undertaken in Ontario, it was found that using a GnRH analog could have much the same effect in hastening ovulation in mares which were in transition as had been demonstrated for those in the midst of estrous activity. The analog is deslorelin.

When estrus was established in mares involved in the study, they were examined regularly by ultrasound. When a lead follicle became 30 millimeters or more in diameter, implants that contained either deslorelin for the treatment group or a placebo for the non-treated group, were placed just beneath the skin of the neck.

The results revealed that there was a shortening of the interval to ovulation of 38.4 hours in the treatment group. The mares treated with deslorelin ovulated at an average of 47.9 hours after treatment. Mares treated with the placebo ovulated an average of 86.2 hours after treatment.

Administration of deslorelin had no adverse effects on pregnancy rates, early embryonic loss rates, abortion rates, or foal vitality.

The results of the above research demonstrates that utilizing these four hormones--either in a natural or synthetic state--can indeed manipulate a mare's estrous cycle and help pinpoint when she will ovulate. The research also indicates that there have been no harmful effects, other than perhaps an increase in twinning with hCG. But, what about long-term effects?

Long-Term Effects Of Treatment

Colorado State researchers decided to take a look at this aspect by examining the offspring of mares which had undergone long-term treatment with progesterone. One of the most beneficial aspects of altrenogest (Regumate) or progesterone administered intramuscularly is that it can provide the progesterone needed to maintain a pregnancy in mares whose bodies do not produce enough of the hormone naturally. It also can be used to prevent non-bred mares from coming into estrus for long periods, something that often enhances performance careers.

In this study, 30 light-horses which had reached two years of age were evaluated. Involved were 15 colts and 15 fillies. The dams of the 30 youngsters had been divided into two groups. One group had received a placebo while the other group of mares had received altrenogest orally from Day 20 to Day 320 of the pregnancy.

The study indicated that altrenogest did not have a negative effect on the offspring of treated mares. Researchers found that the interval to first ovulation, duration of estrus, diestrus, and estrous cycles were similar for treated and control female offspring. In addition, pregnancy rates per cycle and after three cycles did not differ.

For the stallions, it was found that age at puberty was similar for treated and control stallion offspring. In addition, seminal characteristics of the last nine ejaculates prior to castration as well as sexual behavior were not different between the two groups. Testicular weight and size as well as spermatozoal production were not affected by treatment of the dams.

The ultimate conclusion arrived at by the researchers was that treatment of mares with altrenogest during gestation had no effect on reproductive performance of either male or female offspring.

Overall, current research seems to indicate that manipulation of a mare's reproductive process through short-cycling and hastening ovulation can be done in relative safety. However, as with so many other aspects of reproduction, it is a manipulation that should be carried out by trained professionals. It is not something the average horse owner should launch into without hands-on help and advice all along the way by a veterinarian.

About the Author

Les Sellnow

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 or by calling 800/582-5604.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More