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Minerals For Horses

Minerals refer to the various elements that are required for health and to support the systems of the body. Those that are needed in larger amounts are referred to as minerals and amounts required in feeds are expressed as percentages of the feed on a dry matter basis. Those that are needed in much smaller amounts are referred to as trace elements. Amounts are expressed as mg/kg of dry matter, which is the same as ppm or parts per million. The soil supplies the plants, and how much they take up depends on the individual plant’s needs as well as its efficiency at taking them up. In turn, the horse gets what it needs from the plants it eats, and again it depends on how efficiently they do this, their needs and also how available the trace elements or minerals are.

Minerals and trace elements are intricately inter-linked. It is important not only to supply enough of any particular element, but also that it is in balance with those that interact with it. For instance, too much phosphorous will prevent the uptake of calcium but the opposite does not apply, so we would look to provide at least as much as is needed of the phosphorous, and then make sure there is in excess of requirements of calcium.

The elements are not absorbed as the straight metal, for instance, but combined with other elements as a salt or in combination with a protein, which is termed chelated. Horses absorb elements best in plant form as this is how they evolved to utilise elements. This is not how they are always presented.

Minerals.
Calcium: 35% of bone is calcium and this and teeth account for 99% of the horse’s calcium. The other 1% is vital for muscle contraction, nerve impulses, blood coagulation and many enzyme systems. If any of these processes is short of calcium, then it is “borrowed” from the bones. This can happen during lactation, which is very demanding of calcium. Repeated foalings year on year result in a gradual decline in skeletal reserves of calcium. Vitamin D is needed in order for calcium to be absorbed, but this may not be as important in the horse as other species. Phosphorous, Magnesium, phytate (found in cereals and in particular in wheat bran) and oxalate (found in tropical grasses) affect uptake of calcium. When fed in excess of needs, less is digested and more excreted in the urine. Calcium is one of the group making up electrolytes.

Calcium is found in forages and in particular in lucerne. It is inadequately supplied in cereals and pulses. It is absorbed from the small intestine. When diets are deficient in calcium, bones will suffer. Older horses’ skeletons will become weakened and they will exhibit shifting lameness. Young horses will have weakened long bones and enlarged joints. Prolonged, intensive exercise can result in synchronous diaphragmatic flutters (“The Thumps”) and collapse; calcium depletion is implicated alongside dehydration, reduced blood sugar and other components of exhaustion. Calcium may be lost in sweat.

The daily requirement of calcium for a 500 kg horse is 20g per day and up to 60g per day for lactating mares. Growing youngsters need around 40g per day. These are minimum amounts and equal to 0.16 – 0.48% of the diet. So long as sufficient P is available, levels of up to 2% in the diet can be tolerated. This supplies 250g. Lucerne is around 1.5% Ca. Grass is in the order of 0.5%

Phosphorous: Bone is up to 17% P. It is also essential for energy transfer reactions and in the synthesis of various structural components. Excess P prevents adequate absorption of Ca as well as other trace elements. A deficiency of P leads to similar signs as seen with a deficiency of calcium, but more common is an excess of P, leading to calcium deficiency, osteochondrosis, shifting lameness and distortion of the facial bones (Millers Disease). A slight convex shape to the facial bones immediately below the eye may be a sign of past or current calcium deficiency or phosphorous excess.

Phosphorous is found mainly in cereals, where it is in excess of calcium; forages supply sufficient but not more, but generally in balance with calcium. Linseed is a good source, but it is in excess of calcium. The ideal ratio between calcium and phosphorous is cited at 1.5:1 to 2:1. So long as sufficient phosphorous is supplied, it is not harmful if the calcium ratio is higher. Few feeds are deficient in P but beet pulp is only 0.08%P.

Requirement per day for a 500 kg horse is 14g. Youngstock may need 20g and lactating mares nearer 40g, equivalent to 0.12 – 0.3% of the total diet. Forage at 0.25% P will supply 2.5g per kg. Linseed at 0.5% supplies 5 g per kg. Seaweed and Brewers Yeast are 1.2% and 1.5% P respectively.


Magnesium: this makes up 0.05% of the body mass, with 60% of this in the skeleton, 30% in the muscle. Magnesium is one of the electrolytes. It is important for muscle contractions as well as enzymes. Magnesium interacts with calcium. It is found in many feeds and is especially available in lucerne. Absorption is reduced when there is excess phosphorous. It is absorbed from small and large intestines.

Magnesium deficiency is rare and pastures that cause deficiency in cattle do not affect horses. Deficiency will result in nervousness, muscle tremors, ataxia (uncoordinated gait) and mineralisation in the aorta. Tetany in transported horses has been reported, but this may be due to lack of calcium. Excess can cause kidney failure, calcium deficiency (affecting nerve and muscle function, bone integrity) and scouring. Magnesium has been used i/v as anaesthetic and has analgesic and sedative properties. It is used to calm horses, but frequent use may have side effects that have yet to be reported. Its influence in the incidence of rotational falls is under investigation and many dressage riders now avoid it due to delayed response to aids.

Requirement per day for a 500 kg horse is 7.7g – 13g. Forage varies from 0.15% to 0.3% and will supply 1.5g – 3g per kg. Lucerne supplies more than grass. Seaweed, linseed and brewers yeast are all good sources. (0.9%, 0.3% and 0.25% respectively). 0.1% of magnesium in the total diet supplies the maximum requirement. Few feeds are deficient; reports of grazing being deficient are not substantiated. Fertilised high production grasses may be low in Mg when growing rapidly.

Potassium: Another of the electrolytes, potassium (K) is important in maintaining acid balance in the body and is vital in transmitting nerve impulses and muscle contraction. It is supplied by forages, which are generally 1-2% K. Cereals have a lot less, 0.3 – 0.4%. Potassium is excreted through the kidneys and is lost in sweat. During intensive exercise, the gut acts as a reservoir of K, provided feed is supplied appropriately.

Deficiency of K results in depressed appetite so it is important that hard working horses (endurance) are fed to maintain supplies in the gut. Horse sweat is hypertonic compared with blood so K losses (and other electrolytes) in sweat can be high. Hyperkalemic periodic paralysis (HYPP) is a condition in which affected horses are intolerant of high K in the diet. It is an inherited disease of Quarter Horses descended from Impressive. Their diets should be below 1% potassium. Excess amounts are excreted.

Horses require from 25 to nearly 50 g of potassium per day (550kg horse) or 0.2- 0.4% of the total diet. Forage fed horses need no supplementation.

Sodium: the major electrolyte, it is also vital for the nervous system and movements of nutrients. It is essential in maintaining fluid balance. It is the only electrolyte not supplied from the diet in amounts sufficient for strenuous work. It is usually supplemented in the form of ordinary salt (NaCl) as a free access lick. The large intestine acts as a reservoir for this and other electrolytes. Excess is excreted via the kidneys. Use of electrolyte paste to replace losses due to sweating may exacerbate ulcers; if the horse is not drinking freely, dehydration may increase.

If horses do not have enough sodium (as salt) they will tend to lick, eat slower, may lose appetite completely, can become uncoordinated in their gait and drink less. So long as there is sufficient water, excess can be excreted. If not enough water, excess can cause dehydration.

Requirement for sodium is from 10 – 40 g per day, equivalent to 26 – 104g per day of salt. This is equivalent to 0.08 – 0.32% of the total diet. Forages vary from 0.06% to 0.18% but are very variable. Free access to salt is fine for most; in hot weather or in intensive work, salted water can be offered alongside plain (25g common salt per 5 litres, or 1 tablespoon per gallon).

Sulphur: in the form of sulphur containing amino acids (cysteine and methionine), some of the B vitamins, insulin, heparin and cartilage, makes up 0.15% of the body.

There is no record of the effect of deficiency. In ruminants, excess sulphur can induce copper deficiency but not in horses. Through normal diet excess is not possible, but if sulphur, e.g. flowers of sulphur (pure sulphur in a fine powder form) is fed, the horse will be very ill and may die.

Requirements for horses have not been established but are likely to be met by a balanced diet. Estimated requirement is in the order of 15-18g per day (500 kg horse) or around 0.15% of the diet. Forages supply around 0.2%; seaweed 2.9%.

Trace Elements.
Cobalt: As with so many trace elements, there are no figures available for horses, so assumptions are made based on other animal’s requirements. Cobalt is used by gut microbes in the synthesis of Vitamin B12. A deficiency has never been reported and it is likely that horses need a lot less than other species. Requirement may be 0.05mg/kg of diet and will be met by normal feeds. Lucerne has 0.14 – 0.4 mg/kg; seaweed 5 mg/kg.

Copper: This is used in the synthesis of tendons and ligaments, production of pigment and mobilisation of iron. The amount of copper in feeds is very variable. It has been thought that higher levels of copper can protect against OCD but this has not been shown to be true. Copper deficiency can result in loss of pigment, with pink areas appearing around the eyes and on the muzzle. Apparently cream/palomino horses can darken to liver chestnut on a balanced diet following a poor diet although Cu deficiency has not been proven. There may be a genetic link, with some individuals perhaps being poor utilisers of copper. Grey Arabs seem particularly affected with depigmentation.
A level of 8 mg/kg has been suggested as required level, or 100 mg/d for a 500 kg horse. There is sufficient in lucerne (8-9 mg/kg); linseed, 11.5; TopNosh 13. Molasses is high at 66 mg/kg. Grazing may be deficient but many pastures do supply sufficient, contrary to some assertions. Repeated dung removal may also deplete mineral reserves in time.

Iodine: this is important in the thyroid gland for making thyroid hormones, that regulate metabolism. The signs of excess are the same as deficiency – an enlarged thyroid gland (goitre) and limb deformities in foals. Fertility may also be affected. Soya beans and brassicas can in excess cause iodine deficiency. Excess has been reported with inorganic iodine and high iodine seaweed (1,800 mg/kg) but no verified accounts of toxicity from other sources. Seaweed from shallower water than kelp, Laminaria spp, has less iodine (150 – 500 mg/kg) Requirement based on other species is 4mg for a 500 kg horse (0.3 mg/kg of diet). 60 mg could be toxic. This would be supplied by around 35 g of kelp or up to 400g of Ascophyllum nodosum.

Iron: the red colour of blood and muscle is due to iron. It is involved in oxygen transport and some enzymes. Deficiency causes anaemia, with pale gum sand mucous membranes. Forages are good sources, grains poor. Iron levels are high in poorly drained pasture associated with a hard pan. Excess iron can cause zinc deficiency. Some believe it is associated with laminitis but this is very debatable. Supplemental iron seems more toxic than organic iron. True requirements for horses are not known but minimum recommended intake is 400 g for a 500 kg horse, or 32mg/kg of diet. Lucerne 200 – 300 mg/kg. Max tolerated 1,000 mg/kg in total diet. It is very rare for diets to be deficient in iron, so anaemia is unlikely to be due to lack of iron. Bleeding, ulcers, worms and intense fast work all can deplete haemoglobin and thus iron and the possibility of these should be addressed before increasing iron in the rations.

Manganese: used for carbohydrate and fat metabolism and cartilage formation. Forages are a good source; cereals poor. It is important for the proper development of limbs and joints. The precise requirements for horses have not been established, so, based on the needs of other animals, estimated at 400 mg/d (500kg horse) or 32 mg/kg of feed. Excess may interfere with Phosphorous, but no other adverse effects reported. Beet pulp 76 mg/kg; lucerne 40; grass 100 – 200.

Selenium: this is important in maintaining the health of cell membranes and thyroid metabolism. Its level in plants will increase in alkaline soils; it is better absorbed by horses than ruminants. Vitamin E is essential for its absorption. Deficiency shows as difficulty moving and swallowing, weakness, heart and respiratory problems. Links between tying up and selenium/Vitamin E status have been over-stated. Horses are more susceptible to toxicity than cattle, causing blind staggers, hoof problems (the hoof capsule can even become detatched), hair loss, hoof cracks around the hoof. Requirement is 0.08mg/kg of diet (500 kg horse; total requirement 1mg) although this has not been definitely established. Diets should not normally exceed 0.5mg/kg. Lucerne 0.1-0.5 mg/kg; grass very variable; Linseed 0.4.

Zinc: is involved in many enzymes. Insufficient gives more problems to youngstock than older horses, causing loss of appetite, reduced growth, hair loss, parakeratosis (elephant type scaly, thickened skin). Excess zinc causes copper deficiency. Requirement has been estimated at 40mg/kg of the diet in the absence of evidence that less is adequate. If the recommended daily amount for a 500 kg horse is 400 mg then the amount in the diet is 32 mg/kg. Linseed 38mg/kg; Brewers yeast 112; grass 25-50; lucerne 20-30.

Chromium: is important in carbohydrate and fat metabolism and aids insulin effectiveness. It is found in brewers yeast in a very available form. It may help calm horses, but no levels have been established for horses and active supplementation with chromium is inadvisable as it may have as many disadvantages as not. Inadvisable for horses with PSSM.

Fluorine: involved in bones and teeth. Forages contain 2-16mg/kg, grains 1-3. No deficiency has been reported in horses and no requirement established.

Silicon: the second most common element. It is found in bone and cartilage It reduces aluminium absorption and toxicity. No amounts have been established, it is widely available but is not readily absorbed. There is some evidence to suggest that horses supplemented with Silicon were less prone to injury. Dandelions and mint, for instance, are a good source of silicon.

Other trace elements such as Boron, (needed for calcium metabolism) Nickel (role undetermined) and Vanadium (needed in some enzyme reactions but known to be toxic) are known to be essential but amounts are not known and toxic levels have not been established.

Well kept and managed horses are unlikely to need supplementation. The need for supplementation increases when processed feeds are used; fields are comprehensively poo picked and nothing put back; grazing is single or limited variety grass; all herbs are sprayed out; no access to hedgerows; high levels of grains and other high GI foods are used; total intake is very reduced; single species hay/lage is used; over supplementing a single mineral causes deficiency of another; soils are deficient; the horse has a genetic inability to absorb a particular element. Amounts needed are often based on other species and when horses are studied specifically, their requirements are usually found to be less than other species.

© Jane van Lennep/Simple System Ltd.
June 2010.