They say that New York is the “City That Never Sleeps” but having grown up there, I assure you there are some quiet neighborhoods that tuck in for the night. Las Vegas on the other hand seems to have the lights on all the time. I just returned from a four-day jaunt to Sin City for the USA 7s Invitational, an international rugby tournament held under the lights at UNLV’s Sam Boyd Stadium. It was a weekend of great rugby, old friends and the complete upheaval of my inner clock. We’ve been home for over a week and I still can’t seem to get my sleep/wake cycle back in order. The casinos in Vegas are notorious for confusing the senses to keep you awake and feeding money into the slots for hours at a time. With gaming floors that are lit up 24/7 and restaurants that serve Budweiser and buttermilk pancakes all day, its no wonder we never know what time it really is (and when in Vegas, do you really care?). This constant bombardment of photons on my retinas designed to increase the House’s take was a timely reminder of another way we can use lights to alter physiologic cycles to produce a desired end result.

People can give birth anytime of year, with little variation in number of births from month to month but naturally, horses typically foal in the spring and summer. Ever wonder why that is? Its nature’s way of ensuring that foals are born when the conditions are most favorable; warm weather and ample pasture for grazing. But we all know someone who has had a foal in the winter months, and in some breeds its advantageous to have your foals born in the early months of the year. So how do we get around Mother Nature? To get the best answer I went to an expert. Dr. Lauren Greene VMD DACT of McGee Equine Clinic in Townsend, MA. Dr. Greene is a veterinarian who specializes in reproduction (aka: making babies). Boarded by the American College of Theriogenology, in addition to veterinary school, she completed a two-year residency at the University of Pennsylvania, New Bolton Center’s Georgia and Philip Hofmann Research Center for Animal Reproduction. When asked about the mare’s reproductive cycle and how it changes with the seasons, here’s what she had to say:

Dr. Lauren Greene and a patient.
“Unlike human women who have menstrual cycles and are sexually receptive throughout that cycle, horse and many other animals have periods of receptivity known as estrus. The estrous cycle follows a pattern with periods of estrus, also known as heat, interspersed with periods known as diestrus, where mares are not receptive to the advances of stallions.

Horses are seasonally polyestrus long day breeders. This means that there are certain times of the year when they are not having these heat cycles.  Normally horses cycle during days of long daylight and are reproductively quiet (anestrus) during days of short daylight-fall and winter. The majority of horses will enter an anovulatory season (with no distinct heat cycles present) during the fall when natural daylight decreases. This is known as the fall transition. Conversely, during periods of lengthening daylight (naturally in the spring) horses will begin cycling again- known as the vernal (or spring) transition. In natural conditions this ensures the foals will be born during the spring and summer months.

Daylight it believed to be one of the key factors in determining cyclicity in horses. It is widely believed that the retinas of the eyes transmit light information to the pineal gland within the brain, which responds to darkness by releasing melatonin. Melatonin then acts on other parts of the brain (hypothalamus and pituitary) to down regulate the ovarian activity.”

If melatonin sounds familiar, it’s because it gets a lot of press when we talk about sleep aids in people. Melatonin is a natural hormone that help regulates our sleep/wake cycles by making us feel less alert and sleeping when it gets dark outside, our body’s way of telling us to go to bed. Too much light leads to low levels of melatonin and sleep problems. Pills containing melatonin have even been prescribed to treat insomnia. But we’re talking about mares here." “In Indiana, most mares kept under natural conditions will cycle from roughly April through October. Unfortunately many breed registries such as the Jockey Club and the US Trotting Association set January 1st as the official birthday for all foals regardless of their actual birth date. Because of this many breeders try to have their foals born as close to January 1st as possible.”

This date is set to make it easier to calculate a horse’s age when it comes to racing, where horses only race against other horses of the same age.  So a foal born on January 1st 2012 and a foal born on December 31st 2012 are technically the same age and would have to race in the same group as 2 and 3 year olds, but the first foal is nearly 12 months more mature, bigger, stronger and faster. In horse years this makes a huge difference!

“Because of this, and the fact that mares are pregnant for an average of 340 days, the breeding season for many farms starts on February 15th. Again, remembering that naturally many mares will normally start cycling in April there is a bit of a discrepancy here.”

So, how do humans affect this natural pattern?

"It has been established that by altering the amount of light that mares are exposed to, we can mimic the naturally occurring transition periods. By adding additional hours of light a mare is exposed to, we can trick her body into thinking that the daylight length is increasing, thereby stimulating her to start cycling before she would naturally.

"This technique is referred to as “putting a mare under lights.” The length of light is extended to a total of 16 hours by providing artificial light, typically by adding light at the end of the day before dusk. Light must be added for 8-10 weeks for a mare to respond, therefore for a desired start date of February 15th, we must start adding this light December 1st. Mares can be housed individually in lighted stalls with lights on from 4:40pm to 11:30pm, or they can be group housed in a lighted paddock. The amount of light should be a minimum of 10 foot candles, which in normal terms means enough light to comfortably read, which roughly equates to one 100 watt bulb in a 12’x12’ stall.”

So there you have it. The same physiology/technology combo that casinos use to keep us up all night gambling away our life savings can be used to alter the mare’s reproductive cycle, giving us more control over when our foals are born.

Special thanks to Dr. Lauren Greene VMD DACT for her contribution!

If you live in central Massachusetts or southern New Hampshire, McGee Equine Clinic can assist you with all of your equine veterinary needs including advanced reproduction techniques.