The changing of the amount of daylight triggers the hairgrowth. In Indiana we put our showhorses under a four foot shoplight for 16 hours every day. This kept the coats absolutely summer short regardless of the weather. You then spend all your time coordinating several sets of sheets and blankets and hoods to assure they are clean and apprproiately warm. Every evening requires a guessing game to the coming cool factor and what Jammies to put on. I would note that should the shoplight not be on for two days or so and the hair may start piling on like mad. Mares heat cycles are also affected by the addition of flourescent light for a lengthy period of time each day.
One thing that must be said here. When you mess with a horses natural protection from the elements you are then totally repsonsible to take care of them. One show horse was sold to ernest but novice owner. She placed him at a barn where the owner decided this poor spoiled animal needed to be a real horse. He threw him outside in January with no blankets and a coat fit for summer show ring. Poor animal dropped about two hundre pounds before putting on enough coat to stabilize his body heat. Cannot say anything gracious about this man's stupidity.
Personally nature is best...but then I'm not going to Congress this year with a wooly horse!
Am personally glad that part of my horse life is over.
