Building Blocks?

They come in various colors and sizes. Some weigh 50 pounds, others a pound or less. But how do we know which salt and/or mineral block to place before our animals? Should we offer all of them and let the horses choose? Can a horse actually get enough salt and minerals from a rock-hard block to meet his needs? Shouldn't we simply buy feed that already contains the necessary ingredients for a balanced diet?

There are many questions about salt and mineral intake for horses. We have three of the top authorities in the business to help answer them. Harold Hintz, PhD, of Cornell University, is one of the world's leading experts in the field of equine nutrition; Karen Davidson, PhD, is a nutritionist with Purina Mills; and the late M.E. Ensminger, PhD, authored more than 500 scientific articles, bulletins, and features--a number of them on equine nutrition--as well as authoring or co-authoring 21 books. In addition, we will draw on recommendations from the National Research Council's (NRC) guidelines for proper equine nutrition.

The basic question is this: Can a horse get the proper amount of minerals from a hard block?Most of the 50-pound salt/mineral blocks on the market today contain 93-98% salt; the rest is a variety of other minerals. Some, like salt, are needed in abundance, while others, such as cobalt, are needed in only minute quantities.

It's important to note that the mineral concentrations in the blocks are usually much higher than the NRC recommended concentrations in the total diet, because the block is expected to provide just a small part of the daily feed intake.

"It all depends on what the animal is doing," says Hintz. "As an example, if you're dealing with a foal, he would be unable to get enough copper from a trace block (if there were insufficient amounts of copper in the diet). However, copper is only one of a number of minerals required for proper development of the growing horse or in maintaining an active adult equine."

With the help of our three authorities and the NRC, we'll discuss just what those minerals are in categories--macrominerals and microminerals. Macrominerals include sodium, chloride, potassium, calcium, phosphorus, magnesium, and sulfur, and are needed in relatively high daily amounts. Microminerals, or trace minerals, include copper, iron, zinc, manganese, selenium, iodine, and cobalt; these are needed in only minute quantities.

Sodium Chloride--Salt

Sodium is listed first of the macrominerals for good reason. It is an essential diet ingredient that helps cells function properly, is necessary for proper muscle function, aids the digestive process, and helps maintain proper fluid balance. Chloride is involved in forming hydrochloric acid in gastric juices, which is vital to the digestion of protein. In other words, it must be consumed in proper amounts if a horse is to stay healthy. Together they make up sodium chloride (salt), which is an essential part of a horse's diet.

Horses which are stressed in performance disciplines, such as endurance racing, suffer from fatigue and other problems if salt lost through sweating is not replaced. Long-term signs of salt deficiency include loss of appetite, reduced growth of young horses, rough hair coat, and decreased milk production in broodmares.

The prime way in which salt is eliminated from the body is through sweat. Thus, a working horse will have a much greater need for additional salt in the diet than will the horse doing nothing more than lolling about in the pasture. Weather also is an influence, with more sweating occurring in hot weather than in a frigid climate.

Hintz, who gave the Milne lecture on equine nutrition at the 2000 American Association of Equine Practitioners convention, said in his presentation that the NRC's sodium requirement for horses at maintenance status and for pregnant and lactating mares is 0.1% of the diet's dry matter, and 0.3% for working horses. Thus, he says, an 1,100-pound (500-kg) horse at maintenance would need about eight to 10 grams of sodium per day, while the same horse at intense work would need 24-30 grams per day.

He said the NRC, which published its findings in 1989, concluded there were not sufficient data to estimate a requirement for chloride, but later research has produced some interesting findings and suggestions. Recommendations range from 30-40 grams of chloride per day for maintenance, he says, to 115-160 grams for the 1,100-pound horse in an intense exercise program.

Can the horse in an intense exercise program get enough salt from a 50-pound salt block? No, says Hintz. He needs salt as an integral part of his diet. Some commercial feeds, he says, contain 1% salt. However, a non-working horse on pasture likely could get enough salt from a block for his needs.

Potassium--Another Electrolyte

Another important electrolyte is potassium. It assists in, among other things, muscle activity and carbohydrate metabolism. Normally it is not included in trace mineral salt blocks. Most roughages will meet potassium requirements, Ensminger wrote in a section on "Feeding Horses" in the book, Horses and Horsemanship."

In most cases, the requirement for potassium is met by good-quality forage, as forage can contain from 1-2% potassium. If during periods of intense work it is ascertained that dietary forage does not, when taken with grain, meet requirements, there are several commercially available electrolytes that can be top-dressed on feed to make up the difference.

Most good electrolyte mixtures will supply significant amounts of potassium, as well as the other electrolytes, such as sodium and chloride. Although electrolytes also are available for dissolving in water, many people feel adding something to the water that might interfere with normal water intake is not a good idea. It is probably best to give horses clean, fresh water in addition to the "electrolyte-treated" water if a water-soluble electrolyte source is to be used.

Calcium and Phosphorus

Two of the major macrominerals are calcium and phosphorus. Both are essential to the development of bones and teeth as well as metabolism. The NRC recommends that an 1,100-pound horse at maintenance (non-working) level get 20 grams of calcium and 14 grams of phosphorus per day.

Most trace mineral blocks don't contain calcium and phosphorus as such, but some specialty mineral blocks do. One typical block, advertised for horses, cattle, and sheep, contains the following nutrients: Calcium 11-12%, phosphorus 11-12%, salt 11-12%, potassium 1%, zinc 630 parts per million (ppm), vitamin A 100,000 IU (international units) per pound, vitamin D 50,000 IU/lb, and vitamin E 50 IU/lb.

Is this an appropriate way to add calcium, phosphorus, salt, and other minerals to the diet? No, says Hintz. It is far better to feed a balanced ration than to depend on the horse to consume the appropriate amounts from a mineral block.

Magnesium and Sulfur

The final two macrominerals are magnesium and sulfur. They are not required in quantity, but again, the experts have concluded that one is better served to present them as part of the normal diet rather than only in trace block form.

Magnesium can be deficient in a high-grain/low-forage ration. It is important in enzyme systems, Ensminger wrote, and also is essential in proper bone formation and calcium and phosphorus metabolism. Conversely, he added, an excess of magnesium can upset calcium and phosphorus metabolism. The normal trace mineral salt block contains magnesium at the rate of 7,500 ppm (our 1,100-pound maintenance horse needs 7.5 g according to the NRC).

Sulfur is the final macromineral to be discussed. What little sulfur is needed by the horse is normally included in a balanced ration (0.15% of the diet, says the NRC). There are yellow salt blocks on the market that are labeled sulfur blocks; however, these blocks are normally not necessary. Normally, a 50-pound block will contain up to 97% percent salt and 3% sulfur.


One of the most important microminerals, especially for young horses, is copper. It is one of the key elements of hemoglobin formation and also is associated with normal bone development in young horses.

The NRC recommends that a foal's diet contains 10 ppm of copper, but Hintz says some researchers recommend up to 25 ppm. Some studies, he says, indicate that when copper is supplemented to the mare's diet during the last 13-25 weeks of pregnancy, the amount of copper in the foal's body at birth is increased. This will aid proper bone development.

The problem, said Davidson in a recent presentation to a group of veterinarians at the company's headquarters in St. Louis, Mo., is that trace mineral levels are low in mare's milk and become lower during lactation. Copper, she says, is a prime example. The copper level in milk generally starts at 8.3 ppm, and by the fourth month of lactation is down to 2.0 ppm. She believes that this produces a strong argument for appropriate creep feeding of foals very early in their lives in order to provide them with the correct balance of minerals.

We come back to the trace mineral salt block. The normal 50-pound trace mineral salt block will contain only 260-380 ppm of copper. Considering that NRC recommends 120 mg daily for the 1,100-pound broodmare during gestation and lactation (and for a horse at maintenance), can she get enough from a trace mineral block? No, says Hintz. Again, the proper approach is to provide sufficient copper in the ration. The same would be true for foals.

Iron is necessary for the formation of hemoglobin (which carries oxygen in the blood) and also is important to enzyme systems. Iron, too, is low in mare's milk. Davidson says that iron levels in milk start at 9.5 ppm and drop to 4.9 ppm by four months of lactation. This, she says, is compared to the 50 ppm recommended by the NRC for foals (40 ppm recommended for a horse at maintenance).

The average 50-pound trace mineral salt block contains iron at the rate of 6,000 ppm. Again, the better approach, researchers agree, is to ensure that the animal's diet contains appropriate iron rather than depending on a salt block.

Zinc is important for the horse's enzyme systems and for normal protein synthesis and metabolism. Horses with a zinc deficiency often have a rough, dull hair coat and loss of appetite, Ensminger reported.

Like copper and iron, says Davidson, zinc levels are low in mare's milk--about 30 ppm until about three weeks of lactation. They then decline to 21 ppm. The NRC recommendation for foals and adults is 40 ppm. The typical trace mineral 50-pound salt block contains 1% zinc. Experts agree that zinc should be provided in the diet.

Ensminger lists manganese as essential for normal bone formation. A deficiency often is characterized by poor growth, lameness, shortness and bowing of legs, and enlarged joints. He estimated that horses with access to trace-mineralized salt containing 0.25% or more manganese could obtain a sufficient quantity (40 ppm for all horses).

Selenium deficiency is associated with muscle disorders. "I think selenium is the most interesting of minerals," Hintz said in his AAEP lecture. "First known for its toxicity, it was then determined to be an essential nutrient, and now widely acclaimed for the immune response and to be an anticarcinogen. Selenium deficiency has been suggested to be involved in some cases of flexural deformities because of the importance of selenium for normal muscle. It is tempting to speculate that selenium deficiency may also be involved in some cases of osteochondrosis in foals."

In some areas, the soil itself is deficient in selenium. A trace mineral block containing 90 milligrams of selenium per kilogram is on the market; however, most typical trace mineralized salt blocks do not contain selenium. The better approach, experts agree, is to ensure that the proper amount of selenium is included in the horse's diet (0.1 ppm for all horses).

Iodine is needed by the thyroid gland to make thyroxine (which regulates metabolism), reports Ensminger. Some areas of the United States (including the northwestern U.S. and the Great Lakes region) have iodine-deficient soil. Salt blocks with iodine are on the market, but again, the better approach is to provide an appropriate amount as part of the diet (0.1 ppm for all horses).

As for cobalt, horses have a very low requirement (0.1 ppm for all horses). Blue cobalt salt blocks containing not less than 100 ppm are on the market, and it appears likely that they could supply a horse's needs. But in most cases, this wouldn't be necessary.

Take-Home Message

You perhaps can get by with a mineralized salt block for the inactive horse, but for growing foals, pregnant and lactating mares, and working horses, the proper minerals must be part of the regular diet. Your veterinarian, nutritionist, or extension agent should be consulted to find out if there are special dietary needs in your area. Then, know what you are feeding in the way of grass, hay, and grain supplement before you start depending on a block to supply all your horse's mineral needs. 


Lawrence, L. "The Magnificent Seven." The Horse, June 2003, 69-74.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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