Many horses today have wardrobes that rival those of their owners—winter rugs, turnout sheets, rain covers, fly gear, and more. We know these blankets can help keep horses warm, dry, and fly-free, but how they impact some aspects of horse health remain unclear. Take vitamin D, for example: Horses need sunlight to synthesize this vitamin that’s important for bone health. So does blanketing affect their ability to produce it? That’s what a research team from New Zealand recently tried to find out.

Sara Azarpeykan, DVM, PGDip, a PhD candidate at Massey University, in Palmerston North, and colleagues presented their study results in a poster presentation during the 2015 American College of Veterinary Internal Medicine Forum, held June 4-6 in Indianapolis, Indiana.

Vitamin D is key to keeping a horse’s body functioning properly. It helps maintain plasma calcium concentrations and promotes calcium and phosphorus absorption from the intestine. It also helps mobilize stored calcium, with an indirect impact on bone mineralization.

Researchers know that good amounts of vitamin D exist in sun-cured forages. Therefore, horses that consume good-quality hay and have at least some outdoor exposure should be getting plenty of vitamin D. Still, “Equine vitamin D metabolism and factors influencing vitamin D synthesis remain poorly understood,” said Azarpeykan.

She explained that human skin’s ability to synthesize vitamin D3 can be influenced by factors including hair and skin pigmentation, season, latitude and altitude, type of clothing, and sunlight exposure.

Many horses in New Zealand spend much of their t