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Some guidelines from Henry for vitamin & mineral supplementation.
I frequently recommend Moorman's GroStrong loose mineral salt; 4 mg of selenium; and 100,000 IU of Vitamin A for my customer's horses.
Here's some essential information on each.
MoorMan's GroStrong loose mineral salt is also available in a pelleted form called StayStrong that contains probiotics. The retail outlet in this area is Farmer's Coop in Frederick.
I first became aware of GroStrong about 25 years ago when I lived in Florida and I asked some cowboys about their working ranch horses that were unusually fit, healthy, and very surprisingly not bleached out in the Florida sun. When I found out that the horses only ate grass (pretty poor quality in sandy Florida) and MoorMan's GroStrong (no grain or other supplements) I was really intrigued. Since then I got a computer program that compares what you feed horses to the NRC basic requirements. I found that for most grass and/or hay diets that about 2 oz of GroStrong a day would supply all the missing nutrients in safe/appropriate levels (except for lysine ... which can be supplied by a small amount of alfalfa or soybeans). The recommendation is to feed 2 oz a day and also allow free-choice access to the GroStrong. There should be no other salt available. When loose mineral salt is also available free-choice it should not be necessary to feed electrolytes in hot weather and/or heavy work - the electrolytes are already in there.
Some horses don't like the taste of GroStrong at first, and starting out with just a pinch in their feed and working up gradually usually works.
The grass/hay and GroStrong (plus a pound or two of alfalfa if lysine is a concern) provides all the NRC nutrient requirements for an idle horse. This is the best place to start for a balanced diet and is sufficient for many horses, including some working horses. Commercial grain mixes are nearly always balanced to provide the correct ratio of nutrients, but they are more energy-dense than grass/hay. So if more energy is required for a performance horse than grass/hay alone will provide (i.e. they can't eat enough grass/hay to provide the energy they need) you can safely add your choice of horse feed in the amount required to supply necessary energy and still maintain a nutrient balanced diet. Horses stomachs are quite small. Do not feed more than 4 pounds of concentrate at a feeding - they won't be able to use it.
Many people mistakenly think that because they are feeding a 'complete' feed that it contains everything the horse needs. It's important to understand that a 'complete' feed contains everything the horse needs in the correct ratio, not the correct amount. It is also important to understand that 'complete' means that it contains the minimum NRC recommendations, not optimum amounts. Therefore, if a 1000# horse is fed 4 pounds of 'complete' feed and his total dry matter intake is 20 pounds then he's only sure of getting 20% of his NRC recommended minimums.
Selenium is deficient in the soil east of the Mississippi. Commercial feeds have the correct ratio of selenium in their 'complete' feeds, but unless you feed nothing but commercial feed (a horse eats about 2% of his body weight in dry matter per day .... so unless your horse eats 20 pounds of commercial mix for a 1000# horse per day, as some horses on reduced carbohydrate diets may) your horse will not have the required selenium unless you supplement it. MoorMan's GroStrong may supply enough to prevent outright deficiency but will not provide enough for optimum health.
If you are not east of the Mississippi, or if you are in some selenium-rich parts of Arkansas you may want to check first with your vet and get a blood test for selenium. Less than 5% of the US has excess selenium in the soil, and therefore grass, so while it is unlikely that toxicity will be a problem, there are some areas of the US where it can be a concern.
Selenium is probably best known for being toxic, as is everything in excess. However, selenium is very important to good health. It prevents/cures sore suspensory ligaments in the front legs, helps eliminate or reduce allergic reactions, improves the immune response to vaccinations, improves breathing, improves endurance, improves muscle function. Some veterinarians are now recommending selenium to treat 'loose' stifles. A recent human study found that the more selenium a person had the less likely they were to have or develop arthritis of the knee.
As a farrier I can sometimes readily see evidence of selenium deficiency when I trim the frogs - it causes the frogs to have a yellow color. In fact one name for selenium deficiency is 'yellow fat disease' - since there is a high fat content in the frog, that is visible when I trim it. I've found that it generally takes at least 4 mg of selenium per day for a 1000# horse to make the yellow go away and stay away ... and to maintain good health.
Another indication of lack of selenium is standing with the hooves of the front legs behind the vertical -- meaning the front legs are not vertical but sloping back under the body. This is frequently caused by lack of selenium. The horse assumes this posture to load the deep digital flexor tendon in an effort to reduce tension on the sore suspensory ligament. Selenium supplementation will typically fix that in 2 or 3 months (about the same time the yellow goes away in the frogs). Standing behind the vertical on the front legs can also be caused by an effort to shift the weight off of the hind end, usually because of soreness in the hips or hind legs. If the horse does not stand with the front legs vertical after 2 or 3 months of selenium supplementation and the frogs are not yellow, it is likely to be a problem in the hind legs.
The FDA has cautions on selenium supplements that say not to feed more than 3 mg to a 1000# horse. Do not let those warnings bother you. The National Research Council in "Mineral Tolerance of Domestic Animals" (1980) says: "Dietary requirements for selenium range from 0.1 to 0.3 ppm in dry matter..." [that is 1mg to 3 mg for an 1100 pound horse] and "... 2 ppm selenium has produced no unequivocally toxic signs, and this dietary concentration is suggested as a maximum tolerable level for all species." [2 ppm for an 1100# horse = 20 mg per day ... so 20 mg/day is considered the upper level of safe]. There is no need to be concerned about supplementing with 4 or even 8 mg/ day. My 1300# Thoroughbred was on 8 mg of selenium a day for 3 years and blood tests showed his selenium to never go above mid-range of normal levels.
Sulfur competes with selenium for uptake, so do not feed selenium supplements at the same meal as MSM, or Glucosamine sulfate, chondroitin sulfate, etc.
Vitamin A is the most common vitamin deficiency in horses according to "Lameness in Horses" (1987), and "Horse Feeding and Nutrition" (1991). GroStrong should supply enough to prevent outright deficiency, but I find that nearly all horses benefit from an additional 100,000 IU of Vitamin A per day. Signs of vitamin A deficiency include things like deep thrush, a failure or reluctance to shed in the Spring or Fall, susceptibility to infections (including hoof abscesses), a dull look to the eye, weak hooves, and scaly perioples. Vitamin A may help in both the prevention and treatment of founder.
I generally only mention supplementing vitamin A when there are clear signs of deficiency (or degenerative suspensory disease, which I find is usually slowed or stopped from progression with 100,000 of vitamin A per day), but all horses may benefit. In fact, optimal levels of vitamin A (at 1.5 to 10 times the NRC recommendations) have been reported to reduce the incidence of tendon breakdown, as well as increase growth, improve red blood cell metabolism, and improve liver function.
Vitamin A is safe to supplement up to 120,000 IU/day for a 1000# horse that is not deficient, according to National Research Council's "Vitamin Tolerance of Domestic Animals" (1987). With deficiency, the safety margin is much higher. Only actual vitamin A in feeds and supplements need be considered. The 'vitamin A activity' from hay and grass is beta carotene which is essentially non-toxic in any amount. Beta carotene is not converted to vitamin A unless the body needs it - and sometimes not then.
Vitamin A is available cheaply as a generic product called 'A-30' which has 30,000 IU of vitamin A per gram. A rounded teaspoon of 'A-30' provides about 100,000 IU of vitamin A. The product is available in 50 pound bags at any feed mill that mixes their own grains. Two and a half pounds of 'A-30' is enough for a year for one horse, so you may not need a 50# bag. In this area, River City in Williamsport, and I believe Farmer's Coop in Frederick, sells it in smaller quantities, and I usually carry some in my truck for customers.
There are hundreds of supplements on the market -- some helpful, and some not. My suggestion is to first cover the NRC requirements, then supplement with optimal amounts of vitamin A and selenium, plus loose mineral salt free-choice. If a few months of that does not result in a healthy, vibrant horse I'd consult your vet and look for underlying reasons.