More is not always better when it comes to supplementing the equine dietWithout good feed, the best horse in the world will never achieve its full potential. Trainers who want winners search out the best oats and best hay they can buy to give their horses the energy they need to succeed. Most trainers also add feed supplements to their horses' rations in a never-ending quest for that winning edge. Some horses get as many as a half-dozen different scoops of this and pinches of that added to their feed bucket. Too many horsemen also feed supplements with the attitude that, if a little is good, a little more is better.
But can a horse get too much of a good thing? They can if trainers fail to pay attention to the total amounts of certain nutrients that their horses get in their rations. While horses utilize and tolerate amounts of fat-soluble vitamins (such as A, E, D, and K) over a wide range, these vitamins are stored in the horse's body. Fed in excess, they can build up in the tissues and ultimately create a toxic situation. Vitamins A, D, and E are relatively cheap, so over 90% of commercial grains and supplements include them because it looks good to have a long list of ingredients on the label. Water-soluble vitamins, which are less likely to be toxic when overfed, still must be excreted if the horse does not need them, and that puts extra stress on the horse's kidneys and liver.
Probably the greatest danger comes from overfeeding certain minerals. Like vitamins, minerals function together in complex metabolic interactions. The body's ability to fully utilize a particular mineral often depends on just the right amount of a complementary nutrient being available. Overfeeding one mineral may create a secondary deficiency of another, even if the horse is receiving adequate amounts of that second mineral in its ration.
Chelated minerals, so popular in many supplements today, make the problem of over-supplementation with minerals more acute. Chelation hooks minerals to an amino acid or peptide that helps them cross the gut wall better. While only 30%-to-40% of a mineral fed in some other form may actually be absorbed into the horse's bloodstream, as much as 80%-to-90% of the chelated form of that same mineral may make it across. That means a horseman must pay even more attention to the total amount in the horse's diet.
The animals that can suffer the most from an overabundance of vitamins and minerals are foals and youngsters-the very animals that often receive them in the hope they will grow bigger and stronger than previous generations. Instead, excess amounts of vital nutrients are suspected as one possible cause of orthopedic developmental diseases, unthrifty growth, and worse.

Vitamin A
Vitamin A is a fat-soluble vitamin that is abundant in fresh, green forage and quality hay, especially alfalfa. It is also commonly added to commercial grain mixes and supplements. The 1989 National Research Council (NRC) recommendation for vitamin A is that horses receive a minimum of 2,000 international units (IU) per kilogram (2.2 lb. = 1 kg.) of their total ration daily, up to a maximum of 16,000 IU/kg daily. (Some vitamins, such as A and D, are measured in international units, and are listed as such on feed labels.) Excess vitamin A can cause fragile, porous bones in mature animals. Mares overfed vitamin A can produce foals with misplaced eyes and fragile bones.

Vitamin D
Horses synthesize their own vitamin D as long as they have sufficient sunlight. Vitamin D is also abundant in sun-cured forages. The NRC recommends a maintenance amount of 300 IU/kg of total ration daily, up to a maximum of 2,200 IU/kg. Vitamin D is involved with utilization of calcium, phosphorous, and changes of cartilage to bone. Feeding excess vitamin D can upset the calcium/phosphorous ratio in the horse's diet, with the result that calcium gets deposited in muscles, the heart, blood vessels, and other soft tissues.

Vitamin E
Not a lot is known about horses' requirements for vitamin E. Researchers have successfully treated equine motor neuron disease (the equine equivalent of Lou Gehrig's disease) with vitamin E. Vitamin E is also an antioxidant thought to limit free radical damage within the horse's body. Vitamin E utilization is linked to proper selenium levels, so many feed supplements combine the two. Therein lies vitamin E's danger. While no toxic effects of feeding excess vitamin E are known, horsemen may be tempted to feed these supplements to the point where selenium reaches toxic levels.

Selenium
Selenium is a necessary trace mineral which must be fed with caution. Horses need selenium to utilize the vitamin E in their diet, but they tolerate it within a very narrow band. The NRC recommends 0.1 mg/kg of the total daily ration, up to 2.0 mg/kg. Some areas of the country have selenium-deficient soils, so animals there may need more supplemental selenium than horses living in other parts of the country.
In contrast, soils in some Western states, including Wyoming, Colorado, South Dakota, and Nebraska, are very high in selenium. When horses graze on certain plants which concentrate selenium in their leaves, the animals may develop "alkali disease." Manes and tails fall out and eventually the hoof wall sloughs off. Ultimately, the horses develop respiratory problems, become blind, and die-a condition called "blind staggers."

Iron
Many racehorses are given "blood tonics" in the hope that increasing the iron in their blood will boost the blood's oxygen-carrying ability and, thus, the horse's stamina. The NRC recommends 40 mg/kg of feed daily, up to 1,000 mg/kg. One popular blood tonic, fed as directed to horses in training, provides 600 mg of iron daily. Some probiotics use iron fumarate as a carrier. Mix a few supplements into a horse's daily ration and it would be easy to overdose on iron. Iron plays a role in connection with many other minerals in the body. It is not uncommon for excess iron to produce a secondary deficiency of phosphorous, particularly in foals. Excess iron can cause diarrhea, anorexia, erosion of the villae needed for absorption of nutrients from the small intestine, and metabolic problems involving the liver, kidney, heart, brain, and spleen.

Iodine
While iodine is essential to proper thyroid function, excess iodine causes goiters in mares and in their foals via their milk. While most seaweed supplements contain safe levels of iodine, those based on the kelp families can contain excess amounts. The NRC recommends a range of 0.1-to-0.6 mg/kg of the total ration daily. Amounts over 5.0 mg/kg are considered too high.

Zinc
Zinc is another nutrient thought to have antioxidant properties. It is found in good amounts in forage, hay, grain, and many of the products added to commercial grain mixes, such as molasses and dried brewer's grains. It is also included in many hoof supplements. Zinc interacts with numerous other minerals in the body, including copper and manganese. Excess zinc can reduce the horse's ability to use those and other minerals. The NRC recommends a range of 40 mg/kg of feed, up to 500 mg/kg daily. Amounts over that level have produced joint problems in young horses (probably because the excess zinc ties up available copper needed by the growing bones) and arthritic symptoms in mature animals.

Magnesium
Magnesium is concentrated in the horse's skeleton and works with many enzymes in the body. Several products reputed to help calm horses contain large amounts of magnesium. Feeding magnesium to the point where it affects the horse's nervous system means that the horse is also ingesting the mineral at a level that will interfere with absorption of other minerals. Magnesium interacts with copper, manganese, calcium, sodium, potassium, and nickel, so long-term use of calming pastes could produce secondary symptoms of deficiencies or excesses related to other minerals.

Balancing the diet
It is important to remember that not all nutrients fed to a horse actually make it into the animal's bloodstream. Some forms of certain vitamins and minerals are more readily absorbed than others, and the overall efficiency of a particular animal's gut plays a role as well. Still, overfeeding any element in a horse's diet does not do him a favor and probably represents an unnecessary dent in the pocketbook.
The National Research Council publishes information about the nutrient needs of horses based on the latest research data. Smart money uses that information to avoid excess amounts of any nutrient in the horse's ration. Look at the levels of various vitamins and minerals in the horse's grain and hay ration first. Add to those amounts the vitamins and minerals in the supplements the horse receives to see if the totals fall within the acceptable ranges. If not, what the horse is eating may actually be hurting him. Another way to play it safe is to feed no more than two supplements at a time.