If you intend to breed your mare by February, it’s time to start planning to turn on the lights.
Most mares do not cycle during the shorter days of winter. As day length increases in spring, mares enter a transition period. This is the time when the ovaries return to cyclic activity by producing eggs or follicles.
Since mares are stimulated to come into heat by increasing daylight, many breeders use artificial lighting for 8-10 weeks to induce early ovarian activity. Artificial lighting is used to increase the day length to 16 hours. Using this protocol, horses will begin to shed the winter coat within 30-60 days and ovarian activity will commence within 60-90 days. During this time, mares will experience a normal transitional period of erratic follicular development and erratic estrous behavior prior to cycling normally.
In the Sacramento region, I recommend lights be turned on from 4:00pm-11:00pm starting in November. Mares that are normally housed outside should be brought into individual stalls before dark to ensure that they are within 8 feet of the artificial light source. If the mare is normally kept in a stall, it should have adequate window space to ensure natural light during the day.
Research suggests that extending the day length by adding light starting in the late afternoon is better than turning the lights on earlier in the morning and shortening the night length. The length of artificial lighting required will vary with your latitude location and, therefore, the natural day length.
How bright should the light be? The wavelength and intensity of light is as critical as the length of exposure. It is recommended that mares be exposed to a minimum of 10 foot-candles of light during the 16-hour period. The rule of thumb is to make sure it is bright enough to read the newspaper anywhere in the stall. Of course, that can vary based on your vision, so if you want to be more scientific you can test the light intensity with a light meter.
There is a difference in light intensity between a horse stall with dark walls and one with lighter colored walls. A 200 watt incandescent or two 40 watt fluorescent bulbs will generally give adequate illumination in a box stall, if placed within 7-8 feet of the mare.
A 35-mm single lens reflex camera with a built-in light meter can be used to measure light intensity. Set the ASA to 400 and the shutter speed to 1/4 second. Cut the bottom off a styrofoam cup and fit the bottom of the cup over the lens to gather light. Hold the camera at the mare’s eye level. The aperture reading should be equal to or slightly greater than F4.
Many farms will set the stall lights on timers to make certain there is no interruption in the light pattern. Note that leaving barn or paddock lights on for 24 hours a day is not advantageous and 8 hours of darkness is necessary.
Be aware that the other horses in your barn will be impacted by the new lighting schedule. Stallions are also affected by day length and have reduced fertility during the winter. They can also benefit from a similar light program to induce their fertility level. Mares who will be foaling early in the year should also be lighted to ensure that they do not slip into seasonal anestrus after their foaling heat.
Make sure to check with your veterinarian to ensure that your lighting program is in the best interest of all your breeding animals.