Artificial Lighting for Mares

Mares are long-day breeders. This means that the majority of mares in temperate climates are stimulated to come into heat by increasing day light. Their normal ovarian cycle peaks in May to June. 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. During transition, the follicles do not always ovulate due to low levels of luteinizing hormone. Estrus or periods of receptivity, during this period, may be prolonged (15-60 days) or result "in split heats".

Seventy-five percent of mares in a temperate climate, such as Canada, respond to the normal increase in day light as experienced in the spring. The other twenty-five percent of mares living in temperate climates, cycle year round. Horses residing at the equator also cycle year round. Ponies are more seasonal than horses. Natural selection may have favoured a more confined breeding season for ponies which evolved under harsher climatic conditions than horses. This would ensure that foals were born under the best climatic conditions.

The natural breeding period of May to July is the period of highest ovarian activity. Some owners prefer to have a breeding season from January to May. Many farms use artificial lighting to induce an earlier onset of regular ovarian activity. Artificial lighting is used to increase the day length to14-16 hours. Mares which 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. Stalls, where mares are normally housed, should have adequate window space to ensure adequate 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. In practice, the additional day light is usually split and added to both the beginning and end of the natural daylight period. Day length should be lengthened starting 8-10 weeks prior to the desired period for resumption of normal ovarian activity. For a breeding season starting February 15th, the lighting program should be started December 1st.

The length of artificial lighting required will vary with the latitude location of your farm and, therefore, the natural day length at that location. For example, the day length on December 1st at Kapuskasing, Ontario, is 8.7 hours, while at Toronto, it is 9.4 hours. If you want to determine the exact day length in your area, you can calculate it by accessing the following website: You must know the Julian date and your approximate latitude to use this calculation. For example, December 1st is the 336th day of the year and Toronto is at latitude 43 degrees 41 minutes north.

Two systems of lighting are used:

  • hold a constant level of light for 14-16 hours throughout the light stimulation period (e.g. to the 9.4 hours of daylight in Toronto on December 1st., add 6 hours of light in the evening) or
  • increase the day light length in small increments by adding 30 minutes to the length of day at weekly intervals until 14-16 hours of light is achieved (e.g. 9.4 hours on December 1, 10.1 hours December 8th, etc.).

The wavelength and intensity of light is as critical, as is the length of exposure. There is a difference in light intensity between a horse stall with dark walls and one with lighter coloured 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. Various references cite a minimum of 2-10 foot-candles of light exposure at mare eye level. Most references cite 10 foot- candles as a minimum. The Manual of Equine Reproduction describes a practical method of measuring light intensity and differences between light and dark coloured walls can be seen and measured.

In this method, 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. If the aperture reading is equal to or greater than F4 (between F4 and F22), then the light intensity is greater than or equal to 10 foot-candles. An incident light meter intended for photography can also be used in a similar manner to measure reflected light intensity.

Horses will begin to shed the winter haircoat 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.

Stallions are also affected by day length and have reduced fertility during the winter. Stallions should also be subjected to a similar lighting program to induce their fertility level to prime breeding season norms.

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. Therefore, it may be best to routinely start a lighting program for all breeding animals including barren mares, pregnant mares and stallions 8-10 weeks before your desired breeding season. Your veterinarian will be able to advise you on specific cases or problem breeders where hormone treatments may also be useful.


  1. Blanchard T.L., Dickson D.V., Schumacher J., Manual of Equine Reproduction, Mosby, p.20-21.
  2. Rose J. R., Hodgson D.R., Manual of Equine Practice, W. B. Saunders Company, p.257.
  3. Savage N., Notes to OVC Veterinary Students

For more information:
Toll Free: 1-877-424-1300
Author: Dr. Bob Wright - Veterinary Scientist, Equine and Alternative Livestock/OMAF
Creation Date: December 1998
Last Reviewed: December 1998