A: While the protein, energy, and mineral content of hay stays pretty much the same over time, levels of vitamins A and E do decrease. In the case of vitamin A, what are actually lost are carotenoids, which act as provitamins, the most important being beta-carotene. Beta-carotene can be absorbed intact from the digestive tract or converted in the intestinal lining to retinol, which is the active form of vitamin A. Retinol does not occur naturally in plant feedstuffs fed to horses.

The majority of these nutrient losses from hay occur during the initial curing process. Up to a reported 80% of beta-carotene in forages is lost during curing, with an additional loss of about 7% per month of storage. Depending on storage techniques used this loss could be as high as 10%. Therefore a 6-month-old bale of grass hay might have slightly more than 10% of the beta-carotene content compared to that same grass directly prior to harvest.

The color of your hay is a good indicator of current beta-carotene levels. Greener hay will have higher levels than hay that has yellowed. Don’t rely on exterior color for this assessment though; open and assess bales for internal color. Leafiness is another good indicator:   the more leaves hay has the more beta-carotene it can be expected to contain.

Timothy hay loses as much as 60% of its vitamin E when cured in swaths for four days. Other research has shown that alfalfa stored at 30^o Celsius (86 ^o Fahrenheit) for three months incurred vitamin E losses of 54-73%.

The more ultraviolet light the forage is exposed to during the curing, the greater the loss of both beta-carotene and vitamin E. Interestingly, the opposite is true of vitamin D, which increases during curing under ultraviolet light. However, the trading off higher vitamin D at the cost of beta-carotene and vitamin E isn’t considered worthwhile.

All this is to say that indeed the vitamin profile of your now 8-months-old hay is very different than when the farmer first hauled it off the field. The question is, is this a problem for your horse?

Both vitamin A and E play important roles as antioxidants. Most of us know that vitamin A is important for vision. Perhaps its most important roles for most horses are in maintaining epithelial integrity and innate and adaptive immunity. Additionally, in breeding animals, vitamin A is crucial to fertility, and both deficiencies and excesses can have detrimental effects. Vitamin E is responsible for maintaining cell membrane integrity, protecting them from oxidative damage. It has many additional roles, such as humoral immune response, and gene expression.

Despite the amounts of these important nutrients decreasing significantly in stored forages over time, in the case of vitamin A, retinol can be stored in the liver during times of abundant supply for use later when intakes are insufficient. These stores are thought to last up to two-six months.

Horses correctly fed a well-formulated, fortified commercial feed will likely consume adequate amounts of vitamin A. On paper they should also be consuming adequate vitamin E. However not all feed companies use natural sources of vitamin E, which are the most bioavailable. Plus, individual utilization of vitamin E varies greatly by horse. Therefore, I recommend that if you have concerns about your horse’s vitamin E status that you ask your veterinarian to draw blood and test vitamin E levels. Blood is not a particularly accurate way to testing vitamin A status because results don’t account for vitamin A stored in the liver.

Supplemental sources of vitamin A are very common in feeds and supplements, so take care not to over-supplement. The National Research Council (NRC) gives the safe upper limit of vitamin A as 16,000 IU/kg dry mater consumed. However it’s quite likely that this is wrong. Horses consuming fresh pasture—and, thus, large quantities of beta-carotene—would be consuming significantly more vitamin A than that if all the beta-carotene ingested is, in fact, converted to vitamin A. Toxic effects of beta-carotene have not been documented in horses, and this is thought to be because the conversation of beta-carotene to active forms of vitamin A can be reduced when intakes of beta-carotene are high. This is not the case when forms of retinol are consumed.

Excessive vitamin A intake can lead to bone fragility, developmental orthopedic disease, hardening of ligaments (hyperostosis), congenital malformations, and other conditions. While the safe upper limit is high, take care when combining feeds and supplements, especially when the vitamin A is in the form of retinyl-esters.

The daily requirements of vitamin A in horses under varying physiological states is not well elucidated. However the NRC guidelines suggest that a 1,100-pound (500-kilogram) horse at maintenance requires 15,000 IU per day. This increases to 22,500 IU per day for horses in work, 30,000 IU per day for pregnant and lactating mares, and about 10,000 to 15,000 IU per day for weanlings to yearlings.

Feeding carrots is an easy way to add more beta-carotene to your horse’s diet (he’d like that, wouldn’t he?!), because carrots are an excellent source of beta-carotene. One small 5 1/2-inch long carrot provides about 20 mg of beta-carotene!

Vitamin E appears to have a fairly high safe upper limit as well with the concentration set at 1,000 IU per kg dry matter in the diet, a figure based on work in other species. In these species issues with blood coagulation and bone mineralization have been reported above this level. Daily requirements for a 1,100-pound (500-kilogramg) horse are 500 IU per day at maintenance, 800 to 1000 IU per day for working horses, 800 to 1,000 IU per day for pregnant and lactating broodmares, and about 500 to 750 IU per day for weanlings and yearlings. Some studies suggest that these recommended levels for working horses are an underestimate, and I have certainly found this to be the case in some of my clients horses so again I encourage testing especially for horses in work.

Take-Home Message

Providing additional vitamin E to horses on hay diets is recommended especially for working horses. Adding a source of beta-carotene or vitamin A is a good idea as hay ages especially for broodmares.