Mares maintained under natural light conditions (ambient photoperiod) generally do not ovulate for the first time until mid-April or May. In contrast, housing mares under an artificial photoperiod (under lights) will stimulate the mare to develop follicles and ovulate earlier in the year than mares maintained under natural or light conditions.

The decision to house a mare under lights is made by the owner or mare breeding manager. If an owner wants to breed the mare early in the season to produce a foal early the next year, the mare needs to be cycling early in the year.

How does housing a mare under lights cause her to ovulate sooner than mares not housed under lights?

The physiological mechanism by which artificial photoperiod works is through interruption of the normal pattern of melatonin secretion from the pineal gland. Melatonin is secreted during the hours of darkness. Extending the duration of light exposure will reduce the duration of melatonin secretion. The reduction in duration of melatonin secretion will be detected by the hypothalamus within the brain and result in increased pulsatile secretion of gonadotropin-releasing hormone (GnRH). The increase in GnRH pulses will then increase pulsatile secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. The increased pulses of FSH and LH will result in stimulation of ovarian follicular development and ovulation, respectfully.

When should my mare be put under lights?

If the goal is to breed a mare early in the season, mares in the Northern Hemisphere should be stabled under lights beginning on approximately December 1 to advance the first ovulation of the year.

How long does my mare need to be under lights for the therapy to be effective?

The duration from onset of artificial light exposure to ovulation is approximately 60-70 days.

What are the specific light requirements for this therapy to be efficacious in my mare?

During these 60 to 70 days, the light intensity needs to be a minimum of 10 foot-candles (approximately 100 lux). A 100-200 W incandescent bulb or a fluorescent light in a 12-foot by 12-foot stall should be sufficient. The goal should be to provide 16 hours of light and allow for 8 hours of darkness. It is recommended to utilize a timer to automatically during the stall or paddock lights on just before dusk and turn off the lights at approximately 11 p.m. It is not advantageous, and actually may be less effective, to leave the lights on 24 hours per day. Mares respond better when allowed a period of darkness.

Why are some pregnant mares put under lights? Is it safe?

Housing pregnant mares under lights is only advantageous or recommended if the mare is due to foal in January, February or early March and if the goal is to re-breed the mare early in her post-partum period.

There is no need to house mares due to foal in April, May, or June.

The reason for considering housing pregnant mares under lights is that some mares that give birth in the winter, with a relatively short duration of ambient light exposure (more accurately a prolonged duration of darkness and melatonin secretion from the pineal gland) will have limited ovarian follicle development in the post-partum period. This may result in a prolonged period of post-partum anestrus, which would prevent the mare from being re-bred for several weeks or more. Mares due to foal later in the spring (i.e. after the spring equinox on March 21) are less likely to experience post-partum anestrus than mares foaling in the winter.

Housing pregnant mares under lights is safe. A second potential benefit in addition to decreasing the probability of post-partum anestrus is that housing pregnant mares under lights may decrease the duration of pregnancy by up to 7 to 10 days.