Question
Is there a reliable source for minimum requirements and safe upper limits of vitamin A in horses? One source that states the presumed upper safe limit for intake of vitamin A is 16,000 IU per kg of dry matter (16,000 IU/kg DM). Yet every manufacturer’s high-protein supplement contains far more than this limit per pound. Considering that the supplement provides only part of the horse’s vitamin A intake, wouldn’t the use of such supplements result in a dangerous levels of intake? Or would the lower content of the other feeds average out to a safe level? Really, it would make more sense to me to state the upper safe limit in terms of IU per pound or kilogram of a horse’s weight.
Answer
Unfortunately, there is limited research on specific vitamin A requirements for horses at different life stages and exercise; however, it has been noted that vitamin A has an important role in vision, growth, and reproduction. Vitamin A requirements for maintenance, growth, reproduction, and work are 30, 45, 60, 45 IU/kg body weight (BW), respectively, according to the current Nutrient Requirements of Horses, published by the National Research Council in 2007. These values are based on studies that have reported the necessary daily amount to prevent the development of deficiency conditions. Further, some researchers have indicated that 100 IU/kg BW is efficient for all physiological states.
You are correct in that the reported “presumed” upper safe level of vitamin A in the diet is 16,000 IU/kg DM or approximately 7,273 IU/lb DM, assuming an average DM intake of 2-2.5% of body weight. The upper safe limit of 16,000 IU/kg DM breaks down to 320 IU/kg BW/day, whereas toxicity levels have been recorded at around 1,000 IU/kg BW/day. For example, if a 1,100-lb (500- kg) horse consumes 1 lb (0.45 kg) of a ration balancer that contains 20,000 IU/lb, it would supply a total of 20,000 IU of vitamin A. To calculate the upper safe level for a 1,100-lb (500-kg) horse, it would be 16,000 IU times 10 kg intake per day (2% of BW), which is 160,000 IU total. So, the 20,000 in the concentrate is well below the 160,000 recommended as the upper safe limit. Toxicity due to beta-carotenes, the precursor to vitamin A found in green forage, has not been reported, thought to be due to the low conversion rates of beta carotene to vitamin A in the horse’s intestinal tract.
Fresh green grass is the best source of vitamin A, as beta-carotene is found in abundance. It has been found that horses maintained on actively growing pastures can meet their vitamin A requirement, solely from fresh forage. Short growing seasons require supplemental feeding of hay and grain that have significantly less beta-carotene levels than fresh forage, and studies have shown that the horse can quickly deplete its vitamin A stores when fed hay-only diets. This decrease in vitamin A status and the fact that feed manufacturers cannot guarantee horses will have unlimited access to fresh forage has led to the inclusion of supplemental vitamin A in commercial horse feeds.
Commercial equine feeds include an active form of vitamin A that does not rely on the conversion process, as is the case with carotenes. The higher level of fortification of commercial equine feeds is considered optimal for the physiological state, rather than marginal levels needed to prevent deficiency symptoms. High-protein supplements, such as ration balancer pellets, are formulated to be fed at low intake amounts (1-2 lb, 0.45-0.9 kg) but provide complete and balanced nutrition to the horse fed different diets consisting of forage or forage and unfortified feeds. These feeds are formulated to provide a concentrated source of nutrients that are included at the appropriate levels to compensate for any variation or deficiency present in the forage or grain component of the horse’s diet. Ration balancer pellets are to be fed at approximately one-fourth of the feeding rate of a traditional feed, so their vitamin and mineral levels are increased proportionally.
The nutrient levels included in commercial feeds provides a level of safety to ensure the horse receives adequate nutrition when managed under a range of conditions, particularly when the forage component of the diet is of unknown nutritional value (e.g., poor-quality hay or pasture), without leading to oversupplementation and the problems associated with toxicity.
