Vitamin A is one of the fat-soluble vitamins, meaning it is stored up in the body. It is potentially the most commonly deficient vitamin for horses that do not receive commercial feeds or do not have access to green forage.
Roughages that are green, leafy and not too old will contain carotene, which can be converted to vitamin A by the horse, but roughages that are bleached, or have been weathered and are dark and dusty will not contain sufficient vitamin content to be considered. If good green forage is available, the horse will generally have sufficient Vitamin A to meet its needs. However, if a horse is fed poor quality roughage, supplemental Vitamin A is needed.
Since so many horses are fed hay that is stored for many months, most all commercially prepared grains contain Vitamin A to help combat this deficiency. It is important to remember that horses can develop a toxicity situation with Vitamin A, if high-potency vitamin supplements are fed on top of an already well-fortified grain base.
Total Dietary (ie. Hay + Grain) Requirements for 1100lb horse, According to 2007 NRC Guidelines:
- 15,000 IU per day for maintenance horses
- 22,500 IU per day for moderately active horses
- 30,000 IU per day for pregnant/lactating mares
Times that are especially critical for additional Vitamin A are the last 90 days of pregnancy and during lactation. High performance horses and weanlings also need additional Vitamin A.
The most common signs of Vitamin A deficiency are dry, scurfy skin and hair coat, and runny eyes and night blindness are other symptoms. A few studies have been done that have shown night blindness can actually be induced by depriving the horse of Vitamin A and then fixed by adding sufficient Vitamin A back in to the diet. Hooves may be dry and scaly. There’s also a slower resistance to respiratory infections, stress and diarrhea. Toxicity symptoms can include dry hair, anemia, and increased bone size.