Short Cycling Mares

Mares are seasonally polyestrus; this means that they experience regular estrus cycles during late Spring, Summer and early Fall, with none occurring during the Winter. The typical mare cycles regularly between March and October, with each estrous period being an average length of twenty-one days. The mare's cycles are controlled by hormones, which respond to increases or decreases in daylight duration with the onset of the seasons.

The reproductive season begins in the Spring with the increase in natural 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.

Occasionally, horse owners or managers find it necessary to bring a mare into heat at a desired time to facilitate breeding, show schedules, stallion availability, synchronization with other mares, maximize early season breeding dates, or line up recipient mares with donor mares.¹ In breeding operations, manipulation of the estrous cycles of mares allows the breeding load to be spread out.

The Mare's Estrous Cycle

In general, the estrous cycle (or oestrus cycle) is the group of physiological changes that occur in sexually mature female mammals which are brought on by reproductive hormones. The changes facilitate fertility and reproduction.

Common terminology referencing the estrous cycle is as follows:

• Estrous (œstrous in many parts of the world outside North America) refers to the entire cycle
• Estrus (œstrus) refers to the "heat" stage of that cycle when the mare is receptive to the stallion's advances
• Diestrus (diœstrus) refers to the period in between the estrus phases when the mare is not receptive to the stallion
• Anestrus (anœstrus) refers to the complete absence of estrus³

So Just What is Short Cycling..?

The act of short cycling a mare means to shorten the duration of progesterone dominance produced by a functional corpus luteum (CL). For the procedure to be effective, the mare must be normally cycling, have a functional CL that is producing progesterone, and the CL must be mature or old enough to respond to the luteolytic agents available to veterinarians.¹ Veterinarians routinely short cycle mares to cause them to come into estrus at a designated time rather than when nature would dictate. The injection of a particular hormone at the propitious time can cut a mare’s normal estrous cycle in half (or more).²

A cursory run-through of the formation of a corpus luteum is paramount to understand how we can manipulate the cycle. Two major steroid hormones are used to describe the different behavioral periods of a mare’s cycle. The first is estrogen from the cells of the dominant follicle. During the follicular phase or estrus, the dominant ovarian hormone is estradiol. During the luteal phase or diestrus, the steroid hormone produced by the CL is progesterone. Progesterone prevents a mare from showing physical signs of estrus, decreases luteinizing hormone release from the pituitary, and exerts a central calming effect on the physical activity of a mare.¹

Light and Timing

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.²

A normal cycle consists of a dominant 40-50 mm follicle that produces high levels of estradiol from granulosa and theca cell interactions. This follicle responds to a surge of luteinizing hormone (LH) from the anterior pituitary gland to collapse and expel the oocyte (egg), and will then refill with blood from ruptured capillary blood vessels surrounding the now ovulated follicle. This act of ovulation starts a clock. If we label the act of ovulation as day zero (0), then the average mare will be ovulating again in 21 days if she does not conceive and develop a pregnancy.¹

In manipulation of the photoperiod, the literature maintains that perception of day length must be constant from day to day, with the use of automatic timers being of benefit. The use of too much light, however, can cause adverse effects. For example, mares that are exposed to continuous light will not cycle properly.⁴ While the use of artificial lights has the greatest benefit on dry or open mares, recent studies have shown that early foaling mares will also respond to added light.

Chemical Manipulation

The tools typically involved in short cycling mares are two naturally occurring hormones—prostaglandin and progesterone—or their synthetic counterparts.² Several pharmaceutical products are used in the horse industry to maximize conception rates once mares are cycling. Prostaglandin is responsible for regression of the corpus luteum during normal cycles. Mares can be short cycled by treatment with protaglandin F2a or one of its synthetic analogues, if there is a viable corpus luteum present.

Prostaglandins are most effective when used on days 6 through 8 after ovulation, potentially reducing the interval from one heat to the next by as much as eight days. Prostaglandin treatment is most often used on mares that were not bred on the previous cycle and that have a viable corpus luteum suppressing the estrous cycle.²

Another hormone that has been used with some success in breeding management is Human Chorionic Gonadotropin (HCG).⁴ HCG aids in stimulating ovulation in mares. It is most often given at the same time the mare is bred. HCG will normally lead to ovulation of a mature follicle within 48 hours, aiding in appointment breeding and helping mares that tend to develop a follicle and fail to ovulate.

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¹Scofield, D., DVM. What is Short Cycling a Mare? March 05, 2014, selectbreeders.com.
²Sellnow, L. Short Cycling Mares. The Horse, May, 2000.
³Equine-Reproduction.com.
⁴extension.org.

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