Meeting the mineral nutritional needs of foals

Minerals, though they make up only a small percentage of the equine diet, are critical — especially for the health of foals.

To meet its mineral requirements, the foal is first dependent on the concentration of those nutrients in the mare's milk. Prior to weaning, Thoroughbred foals consume about 15 kg of milk daily. Because the mare's need for minerals is likewise critical, equine caregivers should be aware of the mineral composition of solid feeds.

In addition, creep feeding and pasture intake provide some of a foal's early nutrition. Most foals begin to eat solid feedstuffs within weeks of birth. According to the National Research Council's (NRC) Nutrient Requirements of Horses (2007), 1-week old foals spend only about 8 per cent of the day eating grain/hay/pasture, but by 21 weeks spend about 50 per cent of daylight hours eating solid feeds.

Minerals contribute to foals' health in many ways, constituting part of structural entities such as bones and teeth; and playing a physiological role in proper acid-base balance; as portions of enzymes, vitamins, hormones and amino acids; and for physiological functions of certain tissues. These include the roles of calcium, sodium and postassium in nerve transmission, and that of calcium for muscle contraction.

Not only must foals have the proper amounts of individual macrominerals, microminerals and trace minerals, but the minerals' ratio to each other is important, affecting absorption, metabolism and excretion of each other and of other nutrients. The macrominerals include calcium, chlorine, phosphorus, magnesium, sodium and sulfur. Microminerals include cobalt, copper, iron, iodine, manganese, selenium and zinc. Trace minerals include chromium, fluorine and silicon.

Macrominerals

Calcium is among the most essential minerals for foals, comprising about 35 percent of equine bone and teeth and contributing to muscle contraction, blood coagulation, the function of cell membranes and regulation of enzymes, such as stomach gastrin. A deficiency of calcium in the developing foal can lead to osteopenia, a condition characterized by poor mineralization of the osteoid tissue and the probability of enlarged joints and crooked long bones.

Calcium absorption efficiency is around 70 percent in young horses, and is affected by the presence of vitamin D (though vitamin D's regulatory role in horses compared to in other species may be different) and by dietary concentration of calcium, phosphorus, phytate and oxalate. Higher dietary concentration of calcium decreases absorption; increased magnesium increases calcium absorption; while, competitively, higher dietary phosphorus decreases calcium absorption. All this is evidence of the interaction between minerals.

Increased content of oxalate in certain grasses may dramatically decrease calcium absorption from the small intestine to the point of deficiency.

As growing foals are accreting bone, they are in positive calcium balance. As stated by the NRC 2007, a 215-kg foal gaining 0.85kg/d with a 50 percent calcium-absorption efficiency would require 27.2 g/d of dietary calcium for skeletal growth plus 8.6 g/d to account for endogenous losses. According to other study data, the daily calcium requirement for growing horses may be up to 42.6 grams.

Phosphorus makes up to 17 percent of bone and is necessary for energy transfer (ATP and ADP) and for synthesis of phospholipids, nucleic acids and phosphoproteins.

Phosphorus absorption, normally about 35 percent to 50 percent, is decreased by high levels of dietary calcium and by increased amounts of oxalate content of grasses. Phytate phosphorus is poorly absorbed by horses, and has been reported to have little or no benefit in feeding. The NRC 2007 notes that "phosphorus absorption is assumed to be higher by foals consuming milk than it is in mature horses," though it is suggested that "creep feed be fed to nursing foals, as milk may be insufficient in phosphorus (as well as in calcium) for optimal growth of foals."

Inadequate dietary phosphorus produces rachitic-like changes in foals, while excess amounts may produce nutritional secondary hyperparathyroidism. Foals fed excessive levels of phosphorus showed severe lesions of osteochondrosis. A calcium to phosphorus ratio of at least 1:1 is as important as the amounts fed, and with growing foals a 2:1 ratio is even safer.

Even if the diet is adequate in calcium, excessive phosphorus may lead to skeletal abnormalities. According to the NRC 2007, using the proper estimate for endogenous phosphorus losses, a 215-kg foal requires 23.7g phosphorus daily.

The foal body mass is about 0.05 percent magnesium, about 60 percent within the skeleton and about 30 percent within muscle. Besides its critical importance in bone composition, magnesium is an important ion in blood, necessary for muscle contraction and as a component of several enzymes.

Inorganic magnesium sources for foals are as beneficial, as is magnesium found in natural sources. Unlike calcium and phosphorus, magnesium absorption is not affected by phytate or oxalate, though excess phosphorus has been shown to decrease magnesium absorption.

Clinical signs of magnesium deficiency include nervousness, muscle tremors and ataxia, with a potential for collapse, hyperpnea and death. Hypomagnesemia also induces mineralization (calcium and phosphorus) in the aorta.

According to the NRC 2007, a 200-kg foal gaining 1 kg daily would need 1.25 g of magnesium for growth, plus 3g for endogenous fecal losses, a total of 4.25 g/d.

The major intracellular cation, potassium, found mostly in skeletal muscle, is critical for acid-base balance, osmotic pressure, and is the most quantitatively important ion involved in neuromuscular excitability.

According to the NRC 2007, "foals fed potassium-deficient, pelleted, purified diets gradually refused to eat and, therefore, lost weight, became unthrifty in appearance and experienced moderate hypokalemia."

NRC 2007 also states that "for growth of foals with an anticipated mature body weight of 600 kg," the daily potassium requirement is 11 g from 3 to 6 months of age, and 14 g from 7 to 12 months. A 215-kg foal gaining 0.85 kg bw/d, with a true potassium retention efficiency of 50 percent, requires 2.6 g dietary potassium/d for skeletal growth in addition to 10.8 g/d for maintenance.

Sodium, with a growth requirement of 0.85g/kg/bw, is the primary extracellular cation, the primary electrolyte involved in acid-base balance, and is necessary for osmotic regulation of body fluids. Sodium is essential for normal function of the central nervous system, generation of action potentials and transport of substances, such as glucose across cellular membranes.

Sodium depletion results in decreased skin turgor, and causes horses to eat dirt and lick other objects to find it. In addition, sodium-deficient horses show a slowed rate of eating, decreased water intake and ultimately cessation of eating. In acute sodium deficiency, muscle contractions and chewing are uncoordinated and horses have an unsteady gait, decreased serum sodium and chlorine concentrations and increased serum potassium. Thus, it is important that foals be introduced to a white salt block or loose salt lick before weaning. The licking behavior may need to be learned by their dam, which is why pre-weaning blocks would be necessary.

Chloride is an extracellular anion engaged in acid-base balance and osmotic regulation. Chlorine is an essential component of bile and a constituent of hydrochloric acid, necessary for stomach digestion. The daily equine requirement for growth is the maintenance requirement of 20 mg per kg/bw plus 13mg chlorine/kg/d up to 6 months of age (or 0.093 g chlorine x kg/bw), and maintenance plus 5mg/kg/bw/d at six to 12 months of age (or 0.085 g chlorine x kg/bw).

Sulfur is found in the sulfur-containing amino acids (cysteine, methionine), B-vitamins (thiamin, biotin), heparin, insulin and chondroitin sulfate.

Microminerals

Cobalt is critical to the cecal and colon synthesis of vitamin B-12.

Copper is an essential portion of copper-dependent enzymes involved in the synthesis and maintenance of connective tissue, mobilization of iron stores and of the integrity of mitochondria, melanin synthesis and detoxification of superoxide.

Foals are susceptible to copper deficiency and exhibit signs of osteochondrosis and osteodysgenesis. According to the NRC 2007, "when foals were fed a liquid milk-replacer diet containing 1.7 mg copper/kg/dm (dry matter) for 13 to 16 weeks, lameness was observed two to six weeks after serum copper concentrations had decreased to <0.1µg/ml."

Iodine is primarily found in the thyroid gland, necessary for synthesis of thyroxine and triiodothyronine, the thyroid hormones that regulate metabolism. It was reported that foals developed goiter when fed feedstuffs below 0.2 mg iodine/kg/dm, or with only alfalfa hay at 0.6 mg iodine/kg/dm, and that foals returned to normal once fed 2 mg iodine/d for two to four weeks. The requirement is thought to be 0.35 mg/kg/dm, assuming a feed intake of 2 percent of body weight.

Iron is found in hemoglobin, myoglobin, cytochromes and within a variety of enzymes, playing a significant role in oxygen transport and cellular respiration. Iron absorption is higher with deficient rations, but decreases with excess cobalt, cadmium, copper, manganese and zinc.

According to the NRC 2007, "the primary sign of iron deficiency is a microcytic, hypochromic anemia. Although young, milk-fed foals are most susceptible to this anemia, iron deficiency is not a practical problem in foals if they have access to soil." The concern for iron deficiency is especially for those foals in confinement without access to pasture soils.

Manganese is necessary for carbohydrate and lipid metabolism, and for the production of chondroitin sulfate, an important cartilage component.

Selenium, to be fed at 0.1 mg/kg diet dry matter, is a critical constituent of the enzymatic antioxidant glutathione peroxidase, and along with vitamin E essential for membrane integrity and free-radical quenching.

Though a necessary nutrient, selenium toxicity occurs at levels greater than 0.5mg/kg/dm. It may be seen in horses (from regions of the country with high selenium soil content) that exhibit "blind staggers" (blindness, colic, abdominal pain, diarrhea, increased heart and respiratory rate) or "alkali disease" (alopecia, brittle hooves) with a chronic selenium toxicity.

Zinc, a constituent of several enzymes (e.g., alkaline phosphatase) is essential, though quite ubiquitous (in soils, galvanized/rubber buckets). Therefore a deficiency in foals is difficult to produce, but it is characterized by poor appetite, poor growth, skin lesions (parakeratosis, alopecia). Only 50 mg/kg dietary dry matter is required.

Trace minerals

The trace minerals fluorine, chromium and silicon are regarded as "minerals of interest" (NRC 2007), though in each case a definitive requirement for the horse has not been determined. Fluorine is present in most water systems and is known to be beneficial for teeth and bone integrity.

Chromium is noted for its benefit toward carbohydrate and lipid metabolism, "especially as a potentiator of insulin to facilitate glucose clearance," (NRC 2007).

These various minerals are necessary nutrients for growing foals, though much additional research is needed to refine their needs. Additional information is also needed to determine if several other chemical elements may play an essential role in trace amounts within the diet, to effect metabolism, physiology or as structural components of growing horses.

Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.