Vitamin A has a number of roles in the horse's body. The main one, and most well-known one, is the role it plays in night vision.
One form of A combines with opsin to produce rhodopsin. Rhodopsin is the visual pigment that helps recognize the presence of light energy and transform it into a signal that travels the nervous system. This nervous system signal is then what allows the horse to see.
However, A also has other functions in the horse. It regulates gene expression during cell differentiation. Due to this regulation, it is very important in reproduction and the creation of the embryo.
Lastly, A is important to help maintain the innate and adaptive immune response to infection.
Traditional feedstuffs usually fed to horses, such as forages, cereal grains, and plant protein supplements, are relatively low in retinol. However, retinol is present in pro-vitamin A compounds, also known as carotenoids.
One of the most well-known carotenoids is beta-carotene, and it is found in most feed ingredients that are used for horses. It is highest in forages and lower in cereal grains.
Pasture is the forage that contains the highest level while mature grass hays contain the least of the forages.
Corn is the cereal grain that has the most beta-carotene, but it has significantly less than the forages.
Beta-carotene is broken down in the small intestine and liver of the horse to be converted into A.
According to the latest edition of the NRC's Nutrient Requirements for Horses, the maintenance level of A for horses is 30 IU/kg BW. For growth the requirement is 45 IU/kg BW, while for breeding, gestation, and lactation it is 60 IU/kg BW.
Vitamin A deficiency is characterized by night blindness, which has been reported in horses.
However, clinical signs of deficiency (mainly night blindness) are hard to induce in horses, and require very low levels of carotene intake over a long period of time (at least a year or more).
However, impaired growth has been reported in growing ponies deprived of carotene. As a result, it is thought that growth parameters are a more sensitive indicator of carotene deficiency than clinical signs.
It also appears, based on the results of a number of studies, that horses may have an ability to adapt to very low levels of carotene intake, making deficiency of A even less of a worry for horse owners.
Vitamin A toxicity is more of a worry in horses than deficiency. However, it should be noted that toxicity due to beta-carotene intake has never been reported. Toxicity results in fragile bones, hyperostosis (simply the overgrowth of bone), teratogenesis (production of a malformed fetus in a pregnant mare), and other disorders.
It has also been implicated in developmental orthopedic disease in growing horses.