By Kathryn  Sharbrough, DVM

Horses are seasonally polyestrous, which means that they have multiple heat cycles during specific seasons of the year.  Typically, in the Northern Hemisphere, horses cycle from Spring to mid-Fall (April-September) with horses that live closer to the equator cycling slightly longer.  About 15-20% of mares will cycle year round and this will vary from mare to mare and year to year.  As the daylight decreases, most mares enter a transitional period where their estrous cycles become more erratic and then they enter a period of anestrous where they are not cycling.  During this anestrous period, they may still show signs of heat because there is a lack of the hormone progesterone.

Daylength is the major factor affecting cyclicity in the mare.  By artificially extending the day length by adding a light source of sufficient power and duration, we can suggest to the mare that spring is closer than what it actually is, and thus normal cycling can be advanced.  Ambient temperature does play some role, as mares in colder climates that are exposed to artificial lighting programs may still be later to start cycling regularly compared to mares in warmer climates exposed to artificial lighting.  Additionally, by exposing these mares to artificial lighting programs, they will also be triggered to shed their winter coats sooner and this needs to be taken into management considerations.

How much light is needed?

The generally accepted amount of light necessary is what is emitted from a 100 W light bulb (or 10-12 foot candles).  This can be tested with a single lens reflex camera.  Set the DIN (ASA) reading to 400 and the shutter speed to ¼ second, and place a plain white Styrofoam cup over the lens.  The aperture reading should be f4 or f5 if there is sufficient lighting (It should be greater than f4).  A general rule of thumb is that there should be sufficient light everywhere in the stall to read the small print classifieds in a newspaper.

How many hours of light are needed?

Previously, it was believed that the mare should be exposed to 16 hours of light (natural light + artificial light) to stimulate early cycling.  Recent research has shown that as little as 14.5 hours of light (natural light + artificial light) will stimulate the mare.  This exposure to light should be continuous, so if the mare is turned out during the day, she should be brought into a stall and put under lights prior to the daylight dimming.  Also, the artificial light should be added after sunset but no additional lighting should occur prior to sunrise.  Research has shown that adding light at the beginning of the day has no effect on hastening cyclicity.

Other plans for artificial lighting consist of extending the natural daylight by 2-2.5 hours of artificial lighting immediately following sunset.  The time the artificial light is turned on should be adjusted each week so that it keeps pace with the sunset.  Another plan is to expose the mares to a 2 hour pulse of artificial lighting starting 9 hours after sunset.  Again the time the artificial light is turned on should be adjusted each week to keep pace with the sunset.  The artificial lighting can be manipulated through the use of a timer or manual management.

There is a new product that is available called the “Equilume”.  This is a face hood that has been fitted with an eye piece, similar to a hood with one blinker, that emits a blue light to one eye.  The blue light provides the correct wavelength to stimulate the mare and this can be used similar to the previous artificial lighting programs.  Some cons are the cost (~$500 per Equilume Mask in 2015) and it can only be used for 5 months or about 1 season.  However, for large scale farms where daily catching of mares and stabling involves larger numbers of staff and overhead costs, there are benefits to using this system. More information can be found at:

When do I need to start an artificial lighting plan?

At least 60-90 days of phototrophic stimulation are necessary to stimulate the onset of normal cyclicity.  As such, if breeding is desired in February, then the lighting program should begin no later than December 1st.

As few as 3 consecutive days of interrupted lighting can cause a delay in the initiation of normal cycling or even cause the mare to reenter winter anestrus.  Therefore, it is recommended to ensure maintenance of the program once started, as well as until 40 days after ovulation or May 1st.  This way, if the mare does not become pregnant for whatever reason or loses the pregnancy to early embryonic death, the mare will be more likely to return to estrus for re-breeding.

What about my stallion?

It is interesting to note that stallions are also receptive to photostimulation.  While they are fertile year-round, their testosterone levels decrease during the winter months which is clinically evident as a decrease in libido and sperm concentrations.  One consideration about a lighting program for stallions, especially if housed in the same barn or location as the mare, is that unlike the mare, if a stallion is successfully stimulated with artificial light, he may be inclined to show a decrease in libido and sperm concentrations earlier in the year than if he is allowed to follow the natural season.

If you have any questions or concerns about your lighting program, please feel free to contact us at 319-366-6441.