Early Season Breeding: Let There Be Light!

Because of the demands of competition and sales, following the natural reproductive cycle dictated by Mother Nature often doesn't fit into man's breeding program. While Mother Nature's time frame stipulates that the mare should be receptive to the stallion in late spring/early summer to produce a foal when the grass is green and the weather is warm, man, as steward of the horse, often has other plans. Since the light of longer days in spring is a main trigger to the horse's reproductive cycle, extending daily light is a simple way to "trick" mares into cycling earlier.

Normally the longer days of spring and summer stimulate the release of hormones that set the reproductive process in motion. However, much the same effect can be achieved with artificial light. The use of an artificial photoperiod to stimulate the equine reproductive system is not new. It began in the late 1940s, but has become more sophisticated in recent years.

The reasons man wants mares to cycle earlier are many and varied, and they often have little to do with the welfare of the horse. It all begins with the fact that many horses have a universal birth date of Jan. 1 for recordkeeping purposes. Thus, a horse being offered at auction as a yearling the summer after his birth year might actually be 1 1/2 years of age from a chronological standpoint. In other cases, less than a year might have gone by since he was born. The foal born earlier usually demonstrates more physical development, which, in many cases, is desirable to buyers. Also, owners preparing horses for the show ring also usually want all the growth and development they can get by the time spring show season rolls around.

Reproduction Primer

The reproductive process starts with the brain's pineal gland. In a manner of speaking, the pineal gland is the brain's special eye. When days are short and there is much darkness, the pineal gland releases the hormone melatonin, which serves to suppress the reproductive system. When the days lengthen, the pineal gland records more and more light and the amount of melatonin released decreases.

Decreased melatonin sends a message to the hypothalamus that it should signal other parts of the body that it is time for the reproductive process to begin. To carry its message, the hypothalamus produces a chemical signal in the form of gonadotropin-releasing hormone (GnRH; see page 82).

When GnRH is secreted at particular concentrations and frequencies, it stimulates the pituitary gland. This is a small gland located at the base of the brain and connected to the hypothalamus by a stalk that contains both blood vessels and nerves. The stalk serves as a pathway of communication between the two glands.

Once it is stimulated, the pituitary gland secretes two hormones--luteinizing hormone (LH) and follicle-stimulating hormone (FSH). They are carried via the bloodstream to the ovaries of the mare and the testes of the stallion.

As the name implies, FSH in the mare stimulates the growth of follicles in the mare's ovaries. LH is responsible for stimulating ovulation in the mare and supporting the initial stages of corpus luteum (CL) development (conversion of an ovulated follicle into the progesterone-secreting CL).

The old rule of thumb, says Pat McCue, DVM, PhD, Dipl. ACT, of Colorado State University (CSU), was that if a newspaper could easily be read in all corners of the stall, enough light was present to stimulate a reproductive response in the equine. What that means as a practical matter, he said, is that a 200-watt bulb in a 12x12-foot box stall normally will provide sufficient light.

At CSU, McCue says, mares are placed under lights beginning Dec. 1. A timer turns on the light at dusk, and the light remains on until 11 p.m. There is no transitional period, he says. The amount of light and the length of time it is on begin immediately and continue unabated. The important thing, McCue notes, is that the mare not be allowed to exit the stall into darkness during the photoperiod (time under lights) or its positive effects will be negated.

When a mare is placed under lights, McCue says, reproductive activity normally begins in about 60 days.

Mares--How Much? How Long?

There have been questions among researchers concerning the amount of light that is optimum and whether lights should be continued once the mare has ovulated. These issues were addressed by researchers at CSU and reported at the 2000 annual conference of the Society for Theriogenology (reproductive specialty) held in conjunction with the American Association of Equine Practitioners meeting in San Antonio, Texas. McCue was a part of the team that conducted the study.

Involved in the December 1999 research were 42 mares in deep anestrus--the reproductive system had completely shut down for the season. The mares were randomly assigned to one of six study groups.

Groups 1-4 were housed indoors under varying degrees of light in 12x12-foot stalls with occasional daytime turnout. Group 5 mares were housed outdoors, but under the same duration of light as the Groups 1-4. Group 6 mares were maintained outdoors with no artificial photoperiod.

Various foot candles of light were applied. A variety of factors can affect how many foot candles of light emanate from a certain size light bulb, but generally speaking, 10 foot candles will correspond to the light emitted by a 200-watt bulb. However, if the stall is white, the foot candles from a 200-watt bulb would be higher than if the stall were painted black.

Meters to measure foot candles of light are readily available, McCue says, and he recommends that horse owners make use of them if they question whether they are using a sufficient amount of light.

In the study, Group 1 mares were exposed to 2.7 foot candles of light and ovulated, on average, at 84 days of light exposure. Mares in Group 2 were exposed to 9.7 foot candles and ovulated, on average, at 78.4 days. Mares in Group 3 were exposed to 12.7 foot candles and ovulated, on average, at 64.5 days. Mares in Group 4 were exposed to 62.9 foot candles and ovulated, on average, at 58.1 days of light exposure.

Mares in Group 5, kept outdoors, were exposed to only 1.9 foot candles of light and ovulated at 91.7 days, on average.

The control group that was housed outdoors with no artificial light ovulated, on average, at 143.4 days after the light regimen was instituted for the other mares.

Thus, one could conclude that the brighter the light, the quicker the mares will ovulate. That appeared to be true in the CSU study, but McCue cautioned against reading too much into the result involving the ultra-bright light. Most studies, he says, have demonstrated that there is a threshold response and once it has been reached, brighter light has little effect. Normally, that threshold is reached with 10-12 foot candles of light.

In another study, aimed at answering the question concerning the value of continued light stimulation after the initial ovulation, 15 mares maintained indoors under exposure of 10 foot candles or more of light were assigned to one of three groups of five mares each after their first ovulation in the wake of light stimulation.

The first group was maintained indoors and under the same light stimulation as they had been. They ovulated for the second time, on average, 20.4 days after the first ovulation.

Group 2 mares were moved to an outdoor paddock with 1.9 foot candles of light. Three of these mares ovulated within 25 days, while the other two ovulated at 28 and 30 days respectively, for an average of 26.6 days.

The third group was moved outdoors with no artificial light offered. Two of the five mares ovulated within 25 days, while the other three ovulated at 60, 82, and 110 days, for an average of 60.4 days.

Thus, the researchers concluded, mares which are stimulated to ovulate with artificial photoperiods should be maintained under the same light intensity after the first ovulation in order to get regular cycling.

Stallions and Lights

Light normally has much the same effect on the stallion's reproductive system as it does with the mare, except the hormones affect cells in the testes, says Juan Samper, DVM, PhD, Dipl. ACT, of Langley in British Columbia. FSH exerts its effects on the Sertoli cells and LH on the Leydig cells. Sertoli cells function primarily to support spermatozoa development. Leydig cells are responsible for testosterone production.

When a stallion is placed under lights for up to 16 hours per day, says Samper, the result can be earlier-than-normal heavy production of both sperm and testosterone. The danger, he says, is that the stallion's sperm production will peak before the height of the breeding season. This means the stallion might be at the peak of sperm production in February or March rather than during the normal peak of the breeding season in May.

Take-Home Message

Extended photoperiods can stimulate mares to ovulate earlier in the calendar year and have stallions' peak sperm production occur earlier in the year. Whether this is good for your breeding program depends on whether you need foals born earlier in the year, or have more mares to cover in March than May. If using extended photoperiods to move your breeding season earlier in the year would be helpful, talk to your veterinarian to make sure you are doing the program correctly. It's not expensive, and the results can mean the difference between a February and a June foal.


Hormonal Control of Estrus

The hypothalamus gland begins the mare's reproductive season by producing gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to secrete follicle-stimulating hormone (FSH) and begin "warming up" for the 21- to 23-day estrous cycle. The season's first ovulation usually occurs 45-75 days after the initial GnRH surge.

During the cycle, FSH stimulates development of follicles in the ovaries until one or more follicles reach 20-25 millimeters in diameter. Estrogen produced by the follicles stimulates estrual behavior, shuts down FSH secretion, and stimulates the pituitary to release luteinizing hormone (LH).

LH facilitates maturation of the growing, egg-bearing follicle, which culminates in ovulation. Immediately following ovulation, the now-empty follicular cavity forms the corpus hemorrhagicum, which in turn becomes a solid body of luteal cells called the corpus luteum (CL) that produces progesterone. Progesterone is the key hormone in maintaining a pregnancy if the egg is fertilized, and it inhibits the secretion of FSH and LH from the pituitary gland. At this point, the mare goes into diestrus, or out of heat.

If the egg is not fertilized, 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 becomes pregnant, the presence of the conceptus will extend the life of the CL (and its production of progesterone) for 35-90 days, until the conceptus can produce progesterone on its own.


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 www.exclusivelyequine.com or by calling 800/582-5604.

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