Contrary to what many people think, horses don’t grow winter coats because temperatures drop. Rather, it is a response to the length of the day. As days get shorter, horses’ coats get longer. 

This means that some of the “traditional” methods of trying to reduce a horse’s winter coat, such as early blanketing or keeping them in a heated a barn, actually have no effect.

To keep a horse’s coat short (without clipping) many show barns use lighting to artificially lengthen the day and “fool” the horse into not growing a winter coat.

Researchers at Texas A&M University’s Department of Equine Scientists tested the theory that exposing horses to 16 hours of “daylight” (the length of the day on the summer solstice) to find out if it would retard fall hair growth or cause early shedding. The experiment was conducted on 16 horses (yearlings and two year olds) that were randomly assigned to normal or extended day length groups.

The project started October 1 when the extended day length (ED) group started receiving 16 hours of day light per day and the non-extended day length (NED) groups received natural day light only.  All horses were housed in the same non-heated barn and none of the horses were blanketed throughout the project. 

On day 1 the hair on a 1×2 inch square, under the mane, was clipped then shaved to skin level. Hair from these areas was reclipped on days 28 and 56 and measured for growth.  

After 28 days, the two groups showed approximately equal growth. But from there, the differences became obvious. On the last day of the experiment, December 6th, the hair on the NED group was nearly three times longer than the hair on the ED group.

Surprisingly, you don’t need special lamps to achieve this effect: you can use standard incandescent or fluorescent lights placed over, or close to, a horse’s stall. Horses have shown a response with as little as 3 foot candles of light (one foot candle is the amount of light that a birthday cake candle generates from one foot away), but 10 foot candles of light is the standard recommendation. Essentially, if you can read a newspaper from any location in the stall, you have enough light. 

To achieve the effect, horses need to receive 16 continuous hours of light (natural and artificial) and 8 hours of darkness. 24 hours of continuous light doesn’t do the trick; there needs to be a period of darkness. Most barn owners use timers to achieve the desired amount of light.

The other effect of keeping horses under lights is that mares will continue to come into season.