The Effects of Photoperiod on Hair Growth in Horses

As a horse owner, you may be concerned with how fast your horse's coat grows. The animal should not sport a long winter coat if you show year-round. A heavy coat causes the horse to perspire more when working in the fall or winter and requires more time for the horse to cool off. Controlling your horse's hair growth has its place, as long as you compensate for a short coat when the animal is exposed to the cold.

  1. Photoperiod

    • Your horse's photoperiod is longest during the summer months.

      A photoperiod is the amount of time per day that an animal or plan receives light. Your horse's photoperiod is longer in the spring and summer, and then shortens in the fall and winter. Its longest photoperiod would be June 21, the summer solstice and longest day of the year. The shortest photoperiod is Dec. 21, the winter solstice and shortest day of the year.

    Hair Growth

    • Your horse grows a long winter coat as daylight hours decrease.

      Your horse's summer hair starts to shed as its exposure to light -- its photoperiod -- shortens. The horse then grows a new, longer winter coat to prepare for colder weather. The animal's exposure to light begins to increase at winter's end and the beginning of spring, and the hairy winter coat is shed, replaced by a shorter and lighter coat.

    Artificial Photoperiods - Light

    • Keeping a horse in a barn with artificial lights can extend its photoperiod and prevent a hairy coat.

      A study by the Equine Sciences Division of Texas A&M University found that horses' hair growth can be manipulated through artificial light, recreating the photoperiod. Research showed that extending horses' exposure to light to 16 hours a day -- the extent of the summer solstice -- prevented the long growth of a traditional winter coat. The study noted that some transition from night to day is necessary to trigger this response.

    Artificial Photoperiods -- Blanketing and Confinement

    • Increase confinement may not be the best option for your horse.

      Other methods of keeping short coats and preventing horse hair growth during shorter natural photoperiods are increased stalling, often combined with blanketing and putting hoods on the horses. This can cause health issues, both mentally and physically, for your horse. Your horse may not be as happy with increased confinement, may be more energetic during times of exercise and may suffer physically from going from the increased warmth indoors to sudden cold outdoors when taken out to exercise.