old, legally, when it might only be days old chronologically. By starting mares under lights in December, they will have passed through the transitional stage of their annual cycle early, and their ovaries will therefore respond as if it were May in the month of February.
In order to have an effective lighting protocol, mares must be exposed to 16 continuous hours of light every day. Many farms will set the lights on timers so that there is no error involved in forgetting to turn the lights on or off. In order to save on electricity, a program might have the lights go on at 7:00 am. The mares would then be turned outside by 10:00 am and brought back into the barn by 4:00 pm. The lights would be set to be on from 7:00 to 10:00 am (3 hours). The mares have from 10:00 am to 4:00 pm with ambient daylight (6 hours). The lights are then timed to stay on from 4:00 pm to 11:00 pm, when they automatically go out. This adds up to a total of 16 hours of light exposure. Some farms that don’t keep mares in stalls have lights set up in paddocks to go on and off on a timer to ultimately provide the same amount of light.
A commonly asked question is how do you know how much light is enough? A good rule of thumb is if you can read a newspaper in the corner of the stall or the paddock when the light is on, it is adequate to provoke the desired response. A more specific technique would be to measure the intensity of light in all parts of the stall or paddock with a light meter (i.e. borrow one from an electrician).
It is recommended that mares be exposed to a minimum of 10 foot-candles of light during the 16-hour period. Note that leaving barn or paddock lights on for 24 hours a day is not advantageous. Allowing mares 8 hours of darkness is more physiologic and more effective. Many breeders specifically do not want to breed for early foals—especially those thatlive in northern states with severe weather early in the year. Others may not want early foals no matter where they live. It is certainly their choice as to when to begin breeding their mares. But for those breeders who desire early foals and want the best chance of getting mares pregnant early in the season—lights are the answer.
This article is provided as an educational service of the Equine Reproduction Laboratory at Colorado State University.