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.”
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.”
"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.”
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.