Article below: EQUILUME: Lighting for breeding and performance – published in The Irish Field, on 17 November 2017

Regulating a horse’s exposure to light can improve fertility, performance, coat condition and health if understood and applied correctly.

For seasonal breeders such as the horse, the lengthening days of spring have long been associated with increased fertility, enhanced performance, improved coat condition and better health and well-being.

The duration of light exposure and the quality of the light administered are key to regulating these important physiological changes. Understanding this regulation is a fundamental factor to consider in our management of horses for breeding and performance.

The first key understanding is the duration of light exposure. Horses are long-day breeders. Their natural reproductive period coincides with the light-filled days between May and September. A normal winter’s day consists of 16 hours of dark and eight hours of light (short day). A summer’s day consists of approximately 16 hours of light and eight hours of dark (long day).

It is the rapid transition from short day to long day that can be used effectively by breeders to stimulate early reproductive activity and ovulation in mares.


On average, it takes 70 days from initiation of long day light treatment to the first ovulation of the season. For this reason it is recommended that long day light therapy begins no later than December 1st to ensure that mares are ready for breeding in time for the official start of the breeding season on Feb 15th.

For pregnant mares the same holds true. Nature intended that pregnant mares foal during the springtime, from April onwards. The tendency to want to breed mares earlier in the year means that mares now foal at a darker time of year than they normally would in the wild. This has resulted in negative consequences for breeding efficiency including longer gestation lengths, smaller foals and poor post-foaling fertility.

As well as hormones that control reproduction, long day photoperiod stimulates important growth hormones that allow foals to mature in utero, mares to produce adequate milk and colostrum and ensures mares are cycling post-foaling and ready to be rebred.

It is recommended that long day light therapy be initiated for pregnant mares 70 days prior to their foaling due date for best breeding outcomes and optimum foal health.

Stallions too need adequate light duration for optimum fertility. While they can breed all year long, fertility is highest during the longest days of the year. Semen volume, sperm concentration, libido and reproductive behaviour are all significantly influenced by the duration of light. It is recommended that long day light therapy for stallions also be initiated 70 days prior to the time of peak reproductive activity. This is especially important for young stallions facing their first, second or third book of mares.


All light is not equal. Natural daylight spans the visible light spectrum between ultraviolet on one end and infrared light on the other. In particular, daylight contains a very high component of blue light wavelengths. It is for this reason that mammals, such as horses and humans, possess receptors in the eye that are most sensitive to blue light.

These receptors are different from the rods and cones used to translate light into vision and are instead used to convey the time of day to the animals internal biological clock. This biological clock governs the activity of every cell and organ in the body, maintaining important daily rhythms in alertness, metabolism, immune function, muscle function, cardio-respiratory function and regeneration and repair.

Normal indoor lighting falls significantly short on providing the optimum light spectrum for humans and animals to function well. For the body’s internal body clock to work most effectively, stable lighting should replicate the lighting provided by the natural environment as best it can.

This means that:

1. Daytime lighting should be enriched with blue wavelength light.

2. There should be a gradual transition from dark to light and light to dark at dawn and dusk.

3. White light pollution at night, that would disturb the sensitive receptors in the eye and play havoc with the internal body clock, should be eliminated.

4. The lighting system should be easily programmed to provide seasonal changes in day length.