How can I use lights to regulate my horse's hair growth? This actually works, because a horse is signaled to shed or grow a winter coat by the length of the day, not by temperature.
Using lights is a sure fire way to do a few things in your barn - prevent winter coats from coming in, keep your mares cycling throughout the winter, and cause a big electric bill. Some barns choose to keep horses under lights for coat control. Horses under lights won't grow a winter coat, so you avoid clipping and the funny colored hair that sometimes results from clipping.
Recent research from my alma mater, Texas A & M, has concluded that 16 hours of daylight (synthetic and natural) are needed to prevent horses from growing coats. You will have the best success if you do two things - keep this routine year round, and use "full spectrum" lighting. (This is basically a fancy way of saying fake sunlight.) Full spectrum lights are available at most hardware stores, and cost a big more but mimic natural light.
I know barns that use lights, and are set on a schedule to extend natural daylight. So, they are turned on at 5 am or so, off when it's naturally light, on again before dusk, and off at 10 or 11 pm. Timers are great for this task, but you will need to adjust the timers every few weeks as the days lenthen or shorten.
Now - you may be wondering if a little 100W bulb will do. Probably not. You should count on at least two 8' tubes of lights per 12 x 12 stall. (the photo above is two 8' tubes...they come in pairs.)
Your mares will also remain cycling throughout the winter, which is something that you as a horse owner will have to decide if you want.
What have your experiences been?