When it comes to stimulating broodmares to cycle early using artificial lighting, which is more effective — overhead lights or light masks? And why do they work?

In many breeds, all horses are assigned a birthdate of January 1, regardless of when they were born. This means late-born foals can be at a disadvantage in competitions and yearling sales. To counter this, many breeders aim to have their mares foal as early in the year as possible.

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Horses are long-day breeders, meaning they naturally begin cycling when daylight hours increase. Left to natural conditions in the northern hemisphere, mares typically start cycling between late March and early April, with those farther north beginning later than those closer to the equator. Research has shown that artificially extending a mare’s exposure to light can induce earlier cycling. However, mares do not begin cycling immediately — there is a delay of 60 to 75 days after light exposure begins. For a breeding season that starts on February 15, lights should be introduced no later than December 1.

Related: Newborn Foal Care: Essential Steps for a Healthy Start

Light exposure alone, however, is not the only factor affecting when mares begin cycling. Temperature and body condition also play significant roles. Mares in colder climates tend to start cycling later than those in warmer areas, and mares in poor body condition will cycle later than those in good condition. Research indicates that mares need 14.5 to 16 hours of light per day, with the additional light provided in the evening. Typically, this means keeping mares under lights in a barn until about 11 pm. Alternative lighting protocols exist but require more precise adjustments based on sunset timing or shorter periods of light exposure nine hours after nightfall.

So, which is better — overhead lights or light masks? The answer depends on your setup and needs.

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Overhead Lights

• Mares must be kept in stalls or a designated pen for extended periods.
• Additional labour is required for turnout, stall cleaning, and feeding.
• Increased bedding usage and waste disposal costs.
• Higher electricity costs.
• Outdoor catch pens with lights require sufficient light intensity and must account for temperature extremes that could delay cycling.

Light Masks (e.g., Equilume®)

• Higher initial cost and annual replacement costs.
• Mares remain on turnout, reducing labour and stall maintenance.
• Weather conditions can impact effectiveness and mask durability.

Both methods are effective. Choosing the best option depends on factors such as the number of mares you need to cycle early, your facilities, labour availability, and local climate conditions. Evaluating costs and logistics will help determine which system suits your breeding program best.

Thanks to Dr. Peter Sheerin, reproductive expert at Rood and Riddle Equine Hospital, and The Paulick Report, for this information.

Related: Deciding Whether to Breed Your Mare

Related: A Guide to Your Mare’s Smooth and Successful Foaling Experience

Photo: Mares generally start cycling 60 to 75 days after receiving sufficient light exposure, though factors such as temperature and body condition can influence this timing. Photo: iStock/NicolasMcComber