By Dr. Robert Hunter, DVM
Thinking of breeding your mare next year? Now is the time to decide when you want that foal to arrive.
All foals in the northern hemisphere share the same birthday of January 1st. Depending on the potential use for the horse, it may be important to have an early foaling date. If you are planning to show or race the horse, the early foals will have more physical and mental maturity and therefore advantages over those born later in the year.
Most mares do not cycle during the shorter days of fall, winter and early spring. As day length increases in late spring, mares enter a transition period. This is the time when the ovaries return to cyclic activity by producing eggs or follicles.
There are a few steps you should take in December if you plan to breed your mare as early as February.
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. In the Sacramento region, I recommend lights be turned on from 4:00pm-11:00pm starting in December. 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.
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.
Stallions are also affected by day length and have reduced fertility during the winter. Therefore there may be an impact on stallions if they are in the same barn or location as the mare that is on a lighting program.
Make sure to check with your veterinarian to ensure that your lighting program is in the best interest of your breeding animal.