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When breeders think of light therapy for advancing the breeding season of open mares, they may wonder too if light affects pregnant mares or their unborn foals. In the past, researchers reported that foals born from mares exposed to blue light during gestation were slightly different than foals born to mares not exposed to blue light.

“That study found blue light therapy shortened the gestation length from an average of 338.5 days to 330 days and produced foals with lower withers heights and shorter hair length compared to those not exposed to blue light. Blue light foals, however, had similar body weights and took less time to stand compared to foals born to mares that were not exposed to blue light,” explained Catherine Whitehouse, M.S., a nutrition advisor for Kentucky Equine Research.

Shorter withers height suggests these foals may not be developing normally and may not perform as expected once they enter competition. “Published data suggest that young horses with shorter withers heights when they enter training or competition may be at a disadvantage compared to taller horses of the same age,” Whitehouse said.

To determine the effect of blue light therapy on the development foals that were “exposed” in utero, the same foals used in the preliminary study cited above were followed postpartum until one year of age.* Key findings of the study included:

  • No differences in selected blood parameters such as white blood cell counts and glucose levels were observed during the first month of life between the two groups of foals;
  • Heart rate and heart rate variability were also similar between foals that were or were not born to blue light mares;
  • Shorter guard hairs observed at birth in blue light foals were no longer noted by two months of age;
  • At birth and again six days later, foals from mares treated with blue lights had significantly lower elbow-to-carpus distances and shorter withers height. Those differences between the two groups of foals decreased over time until one year of age; and
  • No other differences in morphologic features such as weight, height, carpus to fetlock length, chest circumference, or cannon bone length were noted.

Based on these results, the researchers concluded that “blue LED light treatment of pregnant mares is without detrimental effects on postnatal foal growth and development.”

How Blue Light Therapy Works

The pineal gland, located in the midbrain of horses, controls circadian rhythms, including those responsible for seasonal changes in reproductive cycles. This gland produces the hormone melatonin.

“Melatonin levels increase in the winter (shorter days), preventing the release of gonadotropin releasing hormone (GnRH). Without GnRH, a mare’s ovaries are not stimulated to form follicles, the small sacs that release eggs during estrous cycles. Thus, the ovaries are inactive when melatonin levels are high,” explained Whitehouse.

With increasing day length, induced either naturally or artificially by artificial light, melatonin production by the pineal gland is inhibited. As a result, GnRH levels increase, and the ovaries are stimulated to produce follicles.

Beyond the Lights

Considering the challenges associated with breeding and the lower conception rates of horses compared to other domestic species, great effort is put into the reproductive success of broodmares. To achieve reproductive success, especially if artificially advancing the breeding season, consider the mare’s nutritional status.

“Amino acids, long-chain omega-3 fatty acids, and antioxidants all support reproductive health and immunity. Making small dietary changes using these nutritional supplements can improve the health of both mares and foals,” advised Whitehouse.

Kentucky Equine Research offers each of these types of supplements, including EO-3, a highly palatable marine-derived oil that contains the long-chain omega-3 fatty acids DHA and EPA. Studies show omega-3s improve mare fertility, passive transfer of immunity, and colostrum quality.

*Lutzer, A., C. Nagel, J. Aurich, B.A. Murphy, and C. Aurich. 2022. Development of foals until one year of age when the dam was exposed to blue monochromatic light directed at one eye during late pregnancy. Journal of Equine Veterinary Science:103922.

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