An Integral Part of Equine Nutrition

Mineral imbalances can lead to health issues in horses

By Amy Gill, PhD.

Horses eat a variety of forages and concentrates that contain carbohydrates, proteins, fats, vitamins, and minerals, which
they use for different metabolic processes:

Minerals, which are inorganic molecules also called elements, are derived from the earth. They can be incorporated into living tissue (organic) but eventually return to earth in the inorganic form when excreted by the horse, or as ash once the animal is buried or cremated. Basically, minerals are chemical molecules that are used in a variety of ways but cannot be reduced to simpler substances.

Minerals are essential to the physical and mental well-being of horses. They are components of all cells, including blood cells, as well as nerve, muscle, bone, teeth, hormones, and soft tissue. Many other minerals are an integral part of enzymes that catalyze biochemical reactions such as energy production, metabolism, nerve transmission, muscle contraction, and cell permeability.

Macro and micro

Macrominerals, which have established daily intake requirements, are the elements needed in large concentrations from the diet. They include calcium, phosphorus, chlorine, potassium, sulfur, sodium, and magnesium.

Microminerals, or trace minerals, are required only in minute amounts but also must be obtained from the diet.

Microminerals with established requirements include iron, copper, zinc, iodine, cobalt, selenium, and manganese. Other elements or minerals that can be stored in tissue include some of the toxic metals (lead, aluminum, cadmium, and mercury).

Macrominerals are expressed in the diet as a percentage (%), while microminerals are expressed as "parts per million" (ppm or mg/kg) because they are required in much smaller amounts. Some macrominerals, such as the electrolyte minerals sodium, potassium, and chloride, are required in large dietary quantities to help regulate fluid and acid-base balance. The macrominerals calcium, phosphorus, and magnesium are required to build healthy bone tissue during growth and repair. Microminerals, however, are more commonly incorporated into enzymes that catalyze biological reactions.
Because research to determine mineral requirements in horses is ongoing, new requirements might emerge. For example, chromium and silicon are two trace minerals obtained through the diet. Though established recommendations have not been made for either element, both appear to be important
nutrients for horses because:

Calcium and magnesium

Rations fed to a horse must be balanced for all nutrients and energy, meaning the nutrients from the forage and concentrates meet—but do not grossly exceed—what the horse requires to stay healthy and perform well.

Because minerals work with each other and with other nutrients, imbalances of any mineral can lead to health issues. Think of mineral nutrition collectively as a group. Too little of an essential mineral can lead to deficiency, and excesses can cause toxicities. This is important because, as the intake of a mineral increases past the requirement, the excess amount the horse absorbs and retains can be harmful.

A mineral the horse does not absorb might bind other minerals, decreasing their availability and possibly causing a deficiency of those minerals. Therefore, indiscriminate addition of a single mineral to the horse's diet is not recommended.

The ratio of minerals relative to each other is as important as the total amount supplied in the diet. Inverted ratios can affect the availability of other minerals even if they are ingested in the correct amounts. One of the best examples is the ratio of calcium to phosphorus, which should never be less than 1:1 (recommended is 1.5:1) in an equine diet.

The reason for this tight ratio is because the horse's digestive system requires that a molecule of calcium be grouped with each molecule of phosphorus before the intestinal wall can absorb it. If calcium is not available in sufficient amounts, it will be mobilized from the bone. Over time, an inverted calcium-phosphorus ratio causes bones to become weak and porous and more susceptible to stress fractures and breakage.

Though not recommended, excessive calcium intake can be tolerated in horses. Alfalfa hay contains a high percentage of calcium, but it is fine to feed to all classes of horses because calcium consumed beyond the requirement is excreted by the horse in urine. Weanlings can tolerate a ratio of 3:1 and adult horses 6:1 as long as the phosphorus requirement is met. If this ratio becomes inverted, abnormal bone growth can occur in the horse.

Another example of the importance of having the correct ratio of a mineral in relation to others occurs when magnesium is supplemented to reduce nervousness in horses. Severe magnesium deficiency is rare in horses, but symptoms include muscle tremors, poor work capacity, insulin resistance, excitability, and, in extreme cases, convulsions.

Generally, a horse's common diet provides more than adequate amounts of magnesium, but nervous individuals could respond positively to additional supplementation. However, excess magnesium can interfere with calcium metabolism and reduce absorption, so it is important that the ratio of calcium to magnesium remains at 2.5:1.

The total calcium requirement for a mature, 1,100-pound horse is 20 to 40 grams per day depending on level of work being performed; the magnesium requirement is 6.75 to 13.7 grams per day. If the magnesium level is increased, calcium also might need to be increased to prevent low absorption levels.

While supplementation of minerals to balance the diet of horses is a necessity, indiscriminately adding these nutrients to an already balanced feeding program will increase the chance of producing toxic levels and ratio imbalances.

Mineral availability

Minerals vary in how the horse absorbs and uses them. The bioavailability of a mineral, or its relative availability to the animal, is determined by its interaction with other minerals or dietary components as previously mentioned. The more bioavailable a mineral becomes, the lower the dietary concentration needs to be to meet the daily requirement in the horse.

Additionally, the more highly absorbable the mineral, the less the mineral is excreted to the environment—an increasingly important issue. As new restrictions are imposed on the inclusion of dietary trace minerals in animal feeds, feed manufacturers seek alter- natives that are economical, yet maintain performance and reduce nutrient excretion.
Inorganic minerals traditionally have been used in horse feeds and are listed as oxides or sulfates on feed tags. Recently, technology that combines the inorganic mineral with an organic molecule has become an alternative to using inorganic minerals in feeds. Organic minerals are also called "chelates" or "proteinates."

Chelation involves the attachment of the mineral to an amino acid or other organic component so the two do not disassociate in the digestive system, essentially protecting the mineral so it can be absorbed across the luminal wall of the small intestine intact. For some minerals, a chelated compound is bet- ter than other forms, but for others, absorption is not improved with complexing. For minerals that are better fed as a chelate, absorption is more efficient due to a neutral electrical charge.

Nonchelated inorganic minerals have either a positive or negative charge. Both calcium and magnesium are examples where chelated forms are not any better absorbed than inorganic forms, such as calcium carbonate and magnesium oxide. Both are easily separated, and the calcium and magnesium ions are well absorbed.

Because digestibility of oxides is low, they are inexpensive to feed. Sulfates are slightly higher in digestibility but cost more to incorporate into a ration. Organic minerals, however, require more processing than oxides and sulfates and are the most expensive to use. There are many different forms of organic minerals, and bioavailability varies depending on what type of molecule they are attached to (see box titled "Bioavailabilities of zinc, copper, and selenium sources"). Most commonly, zinc, copper, and selenium have higher absorption as proteinates.

Feed companies often use a combination of mineral sources, but to achieve the most benefit, organic minerals should comprise at least 25% of the total mineral supplementation.

By Dr. Amy Gill, originally published in The Thoroughbred Times 2005