Years ago, chopped carrots might have made up a significant portion of a work horse's diet. In Europe, root vegetables are more likely to be fed chopped or dried to compliment a horse's diet. However, carrots are 90% water which in today's world makes them better suited as treats than a major feed source for most US horse owners.
Vitamin A is a fat-soluble vitamin that is stored in the liver for three to six months. The fact that it is stored makes it less likely that a deficiency will occur. Vitamin A plays a role in night vision, reproduction, and a horse’s immune response. It is also an antioxidant that helps protect cells from damage done by free radicals, which cause oxidation to occur. Free radicals are considered the cause of many illnesses and diseases.
Vitamin A is also often referred to as retinol, retinal, and retinoic acid. Good sources of vitamin A are fresh, green forages and newly-stored alfalfa hay. These are high in beta-carotene, which is a precursor to vitamin A. The carotene is actually synthesized into vitamin A in a horse's intestinal wall. The levels of carotene tend to drop in stored feeds, drought-affected forages, and overly-mature grass forages.
An idle mature horse (1,100 lbs) has a Vitamin A requirement of 15,000 IU while a pregnant or lactating broodmare has about double that requirement. While carrots are high in vitamin A (about 2,000 IU per large carrot), they are also high in water content (90%). So it would take about 8 carrots to meet the daily requirement of Vitamin A for an adult horse at maintenance. If your horse has access to pasture in the summer time, chances are it is more than meeting this requirement and the liver stores will see it through the winter months. If you have a broodmare with a higher requirement, the typical commercial horse feed designed for broodmares will also meet these requirements if supplemented according to feed bag directions. Most of the calories from carrots come from sugar, so if you have a horse where sugar in the diet can set off metabolic issues, then you should stay clear of feeding any treats like apples and carrots. If you are feeding carrots, it is best to feed them finely chopped in a feeder, versus out of your hand. This will help prevent bad habits in your horse, avoid the risk of getting your fingers bitten, and decrease the risk of choke from large pieces of carrot.