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Utilizing Lights to Bring Mares Into Heat During Fall and Winter


By Dr. M.A. Thompson , Willow Bend Animal Clinic

As the length of daylight begins to decrease, your mare’s hormone levels will also begin to diminish. As these hormone levels decrease, many mares will stop cycling until the amount of daylight begins to increase in the spring. Since it is often imperative for horse breeders to have their mares foal early in the year, methods have been developed to aid in this process. One of these methods is to increase the length of time mares are exposed to light during the fall and winter by adding artificial lighting. Utilizing artificial lighting, we can create an environment that allows mares to cycle while the days are short and thus breed them at a time that will bring a foal earlier in the year. Ideally, mares are allowed to experience a short day period to allow her to perform at her reproductive best when the days begin to lengthen. Thus, the common recommendation is to begin increasing lighting to sixteen hours per day on December 1. This will allow the mare to experience short days during late fall and early winter while still giving her the required 60 to 90 days of increased light exposure necessary to begin cycling. Although there is much variability between individuals, most mares are pregnant for approximately 11 months. This leaves the sequence of events to be:

1. Start increased light exposure on December 1.
2. Hopefully mare is in foal in early to mid February.
3. Foal is born in January of the following year.

The amount of light is important. Sixteen hours of light each day is typically considered adequate, though there are several other schedules described that vary from this somewhat. It is also generally accepted that the amount of light should be sufficient to read a newspaper easily in any corner of the stall or paddock. Lastly, it has been shown that adding the light at the end of the day is more effective than adding light in the morning. So, bringing mares in before dusk, and leaving them under lights until around 11:00 pm should be adequate. Your veterinarian can help you work out the details of how much light for how long.

There are a few rules that must be followed for increased lighting to be effective. Once the lighting schedule is started, it shouldn’t be interrupted. As few as 3 days without the increased lighting can set the mare back into winter anestrus and the process must be started over. Also, the lights have to be turned off at the end of the daily session, since continual lighting for 24 hours a day can be detrimental to the mare’s cycle. Timers are most often used to turn the lights off at the appropriate time. Mares in more northern climates may take longer to begin cycling once artificial lighting is started than mares in the south.

There are several other methods to bring mares out of winter anestrus and start them cycling. These involve hormone manipulation or a combination of hormone manipulation and artificial lights. Remember, it is very important to keep in mind that your mare’s fertility is strongly linked to her health. You should consult your veterinarian and discuss your mare’s health needs in early December, if your plan is to breed your mare in February. Your veterinarian can also address any other questions you might have regarding the heat cycle of your mare.

Additional resources for this topic:

Slusher, Steven H., Equine Theriogenolotist, Carolynn Taylor-MacAllister, David W. Freeman, Kathy Anderson. “Manipulation of the Mare’s Reproductive Cycle.”
Penn State College of Agricultural Sciences, “Understand How to Get Mares to Cycle Earlier in the Season:”
Mottershead, Jos, “Inducing Early Cyclicity in Mares:”
Oregon Equine Reproduction Center, “Prepare Your Mare For Breeding:”
Colorado State University Equine Reproduction Laboratory, “See The Light – Advancing the Breeding Season for Early Foals:”
The, “Mobile Blue Light Therapy for Broodmares:”
Meadows, Doyle G., John E. Henton, and Fred M. Hopkins. University of Tennessee Agricultural Extension Service EquiFacts. “Basic Broodmare Management:”

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