Although the days are getting shorter, through the use of artificial lighting, Spring, at least for mares, will start in February"Light, more light!"

-Goethe's last words



It is not the temperature hovering around freezing, nor the searching wind that will find the pinhole in the latest Antarctic wear from Patagonia, nor the incredible damp that makes your bones ache that induces the pall at the prospect of a Kentucky winter. Once you warm up, the seasonal changes can doubtless be considered invigorating (at least by the delusional).

No, it is the long dark spells that sour the mood. Those days when the sun shines are fine, but rare. A month can pass with only a slight brightening of the cloud cover, which during January and February is as thick and impenetrable as that of Venus. The darkness colors all moods in its relentless monotony, a low-grade anxiety only heightened by the brief, teasing flashes of unfiltered sun.

The "winter blues" now have a more formal name; Seasonal Affective Disorder (SAD). Each winter, millions of people suffer from more or less the same symptoms. These include a general feeling of malaise, a desire to sleep longer, an increased appetite for carbohydrate foods (and concomitant weight gain), and often a lack of libido.

The onset of these symptoms appears to vary with latitude, beginning earlier in the year in the North and lasting longer in St. Paul, Minnesota, than Atlanta, Georgia. Often, people will go on vacation in the Caribbean and quickly find respite from many or all of the symptoms, with relief persisting for a while even after their return to darker climes. Just as interesting is that the same environmental clues that produce depression in many people are responsible for the seasonal anestrus patterns in mares. Both conditions are tied to the stimulation of biological clocks residing in all animals, whose timing is set by the length of the day.



Around the day

We are normally only aware of our own biological clocks when their timing is disrupted. Wiley Post (the famous aviator who was killed along with Will Rogers in a plane crash in Alaska) was the first person to recognize the symptoms now known as jet lag. Flying around the world in eight days in the early 1930s, Post recognized the adverse effects time zone displacement had on his flying proficiency and struggled to evade them. It has now been determined that the human body can accommodate speeds of about 100 miles per hour across time zones. This is why you do not suffer from "car lag" from cross-country automobile trips.

Just as there are 24-hour rhythms, there are biological cycles occurring on monthly (lunar) intervals, as well as on an annual scale. It is this seasonal calendar within man and horse that is affected by the waning days, and the long cold nights that accompany them.



Pining and the pineal

Deep inside the brain of all animals is a small organ called the pineal gland. In humans, it is the size of a small pea and shaped like a pine cone (hence its name). Also called the "third eye" (because of its nerve connection to the other two), early anatomists were at a loss to describe a function for the pineal. Rene Descartes believed it to be the seat of the soul, since it was the only structure in the skull not directly connected to the brain. Early in this century, scientists had concluded that it was a vestigial organ like the appendix and no longer had a function in the body.

Then some industrious researchers removed the pineal glands of 200,000 cattle, ground them up, and squirted the juice on tadpoles to see what would happen. (I am not making this up.) The tadpoles' skin lightened, so they called the pineal hormone "melatonin."

Naturally, it is unlikely that all mammals produce hormones just to lighten the complexions of self-conscious amphibians. Further study determined that the melatonin was produced by the pineal in direct proportion to the amount of darkness the animal perceived in a day. The longer the "night," the more melatonin was produced. It appeared that melatonin had an inhibitory effect on hormone production by the pituitary and hippocampus. These hormones normally stimulate sexual activity. The exact way melatonin works is unclear, but when an animal has extended high levels of it in the blood, it will undergo shrinkage and reduced function of the ovaries or testes. In effect, melatonin suppresses sexual activity and receptiveness in animals.



Not tonight, honey

Since reproduction of the species is the raison d'tre of each individual (at least as far as the species as a whole is concerned), what, then, is the significance of suppressing sexual activity? Natural wisdom dictates that you should not have young if you cannot feed them (a verity no longer understood by many people). Periods of darkness are associated with winter, which is obviously a period of reduced feed stores and harsh environmental conditions. Other factors, such as food intake and temperature, also help establish the biological clock. These zietgebers ("time givers") are useful to some extent, but are subject to considerable variation from year to year. However, barring cataclysm, daylight remains constant from year to year.

The need to limit breeding to a specific period of time is particularly important for a large animal such as the horse, which has a long gestation period. Eleven months after conception a mare will foal, so timing the onset of the first season's estrus to the lengthening days of spring will find the foals arriving at a time when pasture is becoming more plentiful.



Fooling Mother Nature

Thoroughbred breeders originally respected the natural breeding season of the horse. In 1751, May 1 was proclaimed as the official universal birthday for foals in England. This effectively meant that the breeding season began on June 15, which coincided well with the period horses had increased natural fertility. However, as May fell within the racing season, it led to confusion among racing administrators that had to change horses' ages at midseason. As a result, in 1833, the English Jockey Club decreed that henceforth January 1 would be the universal birthday.

Since that time, February 15 has marked the beginning of the breeding season. But, practically, the true breeding season began around mid-March for physiological reasons and ended in June for practical reasons. This left a fairly narrow window of opportunity for breeders to work, which did not change until around 25 years ago.

Investigators looking into practical applications of the effects of light on the pineal (and, hence, sexual activity) soon discovered that hens in confinement exposed to lights at night would keep laying all winter. As early as 1947, researchers had looked into the effect increasing the winter photoperiod would have on equine estrous behavior. But it was not until the late 1960s that these regimens would be perfected and begin to be widely used.

Various lighting intensities and protocols were tried. Lighting was begun in mid-November, with the hours gradually increased by so many minutes each week until, by February, 16 hours of daylight was reached. Some experiments looked at the efficacy of splitting the light treatment between post-sunset and pre-dawn exposures. Other researchers tried to determine what day length stimulated the greatest response.

Essentially, mares were found to respond best to light added on to the end of the day. At least 14.5 hours of day length, for around 60 days, are needed to get most of the mares through the transitional phase and ovulating good follicles. It was found that more than 16 hours was unnecessary, and that constant lighting has a negative effect. There also seemed to be no reason not to begin the full 16 hours from the beginning.

Although photoperiod was the most important factor in beginning regular estrous cycles in mares, other factors also were found to be important. Stress, poor nutrition, poor condition, and severe cold can all delay or negate the possible advantages of putting mares under lights. But if mares are on an ascending plane of nutrition and are healthy, then most of them will be ovulating by February 15.



All together now

During the breeding season, timing is everything. It is possible to some extent to control the day a mare will ovulate. Mares are given daily injections of progesterone for ten days. On the last day, they also receive a shot of prostaglandin. The result is that nine days after the last shot the mares will begin to ovulate. Almost all the mares will ovulate by day 13.

This treatment has allowed breeders to intensely manage their barren and maiden mares early in the season; in most instances getting them in foal on that first cover. But it also has one drawback. As it became more widespread in Central Kentucky, opening day at the breeding shed was more like a stampede. Many farms have taken to synchronizing their mares in groups throughout the first two weeks of the season, hoping to beat the rush for the more popular stallions.



Is it light enough?

Obviously, there are disadvantages. Keeping mares under lights is more expensive than more traditional methods. Besides the cost of the electricity, there are the expenses associated with keeping mares in the barn. Some have tried to circumvent this somewhat by either keeping the mares in small, well-lighted pens outside, or by turning the mares out after the lights go out. These solutions pose other problems, but can work well some years. When it is very cold, I have found that mares respond better by being kept out of the weather. Although this seems particularly true for maiden mares, most mares seem to respond better if shielded from the cold.

It is also important to make sure that the light is sufficient to stimulate the desired response. A 200-watt bulb will normally provide adequate light, but a simple rule of thumb is that you should be able to easily read a newspaper in the darkest part of the stall.

Another interesting solution was tried at one university. Researchers attached small lights onto the cups of blinkers worn by the mares. Although the mares did cycle properly, they were functionally blind when the lights were on, an obvious disadvantage. But this may work in some situations.



The blue mare

Anyone who has watched horses for any period of time will agree that trying to determine if a mare is depressed is difficult. Mares spend long periods of time (when they are not grazing or huddling beside a wind break) staring off into space. Whether they are suffering ennui or just reliving old races or remembering past stallions is hard to determine.

People suffering from SAD have been found to respond well by extending day length through the use of lights. Many wonder how the Inuit and other tribes, who have lived for thousands of years in the land of the "Midnight Sun" and its months of winter eclipse, handle SAD. Most interesting is that annual birth rates among the Inuit peak in March, which matches closely the melatonin patterns measured in their blood over the year. In effect, the Inuit have a breeding season, regulated by the extremely long, dark nights of winter.