Vitamin D is one of the fat-soluble vitamins, meaning it can be stored up in the body. It is often referred to as the “Sunshine Vitamin”, since the easiest way for both you and your horse to get your daily dose is a little time in the sun. Horses receiving a few hours a day of sunlight will meet their needs easily, and they also receive a dose from eating hay that has not been stored for more than six months.
It is fairly common to see a small amount of Vitamin D added to horse feeds, mainly to help those horses who are stabled and receive little or no turnout time and are often fed hay that has been stored for a lengthy period of time.
Vitamin D plays a role in helping calcium to be absorbed. Deficiency may result in rickets and weak bones. However, excessive Vitamin D may be toxic and may cause abnormal calcium deposits, which could contribute to conditions like spavins, ringbone and sidebones.