Minerals for Horses: Magnesium and Iron

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 Written By Dr. Kris Hiney

This month we continue on our path of discussing minerals required by horses. We will actually be mixing a macro-mineral (magnesium) and trace mineral (iron) together. However, our goal has been to visit minerals in the order of their level of concern by the horse owner and their frequency of need for supplementation.

Function of MagnesiumMagnesium is an important mineral involved in many enzymatic reactions (as so many of the minerals are). The largest store of Mg in the body is in bone (60%), with a relatively small amount present in the blood. Magnesium also acts as an electrolyte as well and therefore is essential for normal nerve and muscle function. Magnesium is a key component in enzymatic reactions involving the synthesis of nucleic acids, protein, carbohydrates and lipids. Magnesium and ATP are almost always found complexed together in the body. In all, Mg is involved in over 300 enzymatic reactions.
Mg requirements Most equine feeds contain Mg in a range between 0.1 to 0.3%. If we use our average horse of 1100 lbs (500 kg), that horse would receive between 10 and 30 g of Mg per day if he was consuming 2% of his body weight per day. Remember, that is our normal goal for feeding. Currently the recommendation for Mg intake for a mature horse is 7.5 g of Mg/d for a 500 kg horse, or 15 mg/kg of body weight per day. Therefore, most horses easily meet their Mg requirements by consuming their regular diets. For young growing horses, the requirement on a percent of their body weight is higher (approximately 21 mg Mg/kg), due to their need to accumulate Mg in growing tissues. As their growth rate tapers off, the concentration of Mg in their diet can be lowered. Pregnant mares do not actually have a much higher Mg requirement than a maintenance horse, and suggested to only be 15.3 mg/kg of body weight per day. Therefore mares should easily be able to consume that amount. For example a 500 kg mare would need 7.7 grams of Mg per day. Only 200 mg more than our maintenance horse! From our above description of the normal Mg content of feeds, this would be easily obtainable by the mare simply eating 2% of her body weight. Remember if allowed, mares will consume much more feed than this! Once our mare has foaled and begins to produce milk, her requirements will increase in order to support her growing foal. During peak lactation she will need 22.2 mg of Mg/kg of body weight or 11.1 g total for that 500 kg mare. Again, this is probably easily obtainable unless she is consuming a marginally deficient diet.Now let’s shift our focus to the exercising horse, who we would expect to have a greater Mg requirement due to sweat losses. To account for changes due to sweating, assume an increase in consumption of 1 – 2 g per day, depending on sweating rate, for a light to moderately exercised horse. If the intensity of the exercise increases and thus the accompanying heat load, the requirement of Mg is suggested to double. Therefore, for those 3 day eventers or endurance horses, they should receive 30 mg/kg body weight or 15 g of Mg per day. It is possible for these horses on a normal diet to end up being deficient in Mg. In several studies in young growing exercised horses, it appeared bone Mg deposition was greater when horses were fed diets higher than previously recommended in Mg. However, in these studies, horses were also fed more Ca which may have led to greater bone formation and thus the accompanying increase in Mg retention. However, because of this interaction between both exercise and growth in the young horse, the Mg requirements of these horses should be treated more like an intensely exercised horse, or 30 mg/kg body weight.
Magnesium deficienciesTypically, acute Mg deficiencies in horses are quite rare, compared to the relatively more common occurrence in cattle. In horses, Mg tetany (or a bout of muscle contraction causing locking of the muscles) has occurred in stressed horses (typically transported) and in lactating mares. Presumably in transported horses, there would have been increased Mg losses in feces (increased intestinal motility while being nervous) or through sweating. In mares, a loss of Mg in milk while on a Mg deficient diet would contribute to an acute Mg deficiency. Rapidly growing pastures are typically low in Mg and high in K may be a risk factor in Mg deficiency, but this is much more of a concern for cattle than for horses, which absorb Mg more efficiently.Some individuals have suggested that magnesium should be supplemented to horses presumed to have insulin resistance. Magnesium does play a role in insulin release by the pancreas and its activity. This idea presumably originates from data in humans. Frequently those with type II diabetes (25-35%) have lower serum magnesium levels than those without the disease. The hyperglycemia associated with type II diabetes may result in increases loss of Mg through the kidney. It is not known if the hypomagnesia is a consequence or a potential cause of type II diabetes and insulin resistance. Studies in humans using Mg as a potential treatment of type II diabetes and insulin resistance have yielded conflicting results, with some having positive results and some with no change in insulin and glucose homeostasis. To date, no such studies have been performed in horses.
Function of IronIron (Fe) is most commonly known for its role in hemoglobin and oxygen transport. It is also a very important ion in the electron transport chain, carrying electrons in order to produce ATP. Thus it is integral in body function. Most equine feeds range between 100-250 mg of Fe/kg of feed. Grains may be lower than forages. Iron absorption in the diet is highest in newborn animals, and also in animals that are fed Fe deficient diets. The body simply becomes more efficient out of necessity and therefore absorbs proportionally more of the iron in the diet.The quickest observable sign of iron deficiency is anemia. Ironically, most animals will never be deficient in iron if they have access to soil. So horses grazing pastures should be adequate in Fe status, but horses which are continually stalled may be at a higher risk of Fe deficiency. Supplementation of Fe has been unable to show any change in hemoglobin or the oxygen carrying capacity of the blood, and therefore may be unwarranted in horses fed normal diets. Over supplementation of Fe has been reported, with clinical signs of iron toxicity disappearing after withdrawal of the supplement. Horses in this study were being fed 0.6 mg/kg Bw/d of ferrous sulphate. Thus owners should be careful about being too enthusiastic in their supplementing regimens. However, others have found no ill effects from feeding horses 500 and 1000 mg/kg feed although serum and liver Zn were reportedly lower. Supplemental iron may be toxic to young foals due to their greater efficiency of absorption. Even when fed at the rate of manufacturer’s suggestions for adult horses, death can quickly result.The requirements for iron in the mature horse are estimated at 40 mg/kg DM and 50 mg/kg for young foals, pregnant mares and lactating mares. For our 500 kg horse, then they should consume 400 mg of Fe per day. Remember when nutrients are listed on a concentration basis, it is assumed that the horse would be consuming 2% of their body weight per day. If our horse was eating 10 kg of feed ranging between 100 and 250 mg Fe/kg, he would receive 1000 to 2500 mg of Fe per day (or 1 – 2.5 g) of Fe. Again, most horses do consume adequate Fe. In a fairly recent study looking at blood mineral profiles, the Fe status of horses with pica (consumption of unusual objects including soil) was lower than in horses who did not perform this behavior. That would make sense as soil does indeed provide the normal grazing horse (they pick up soil inadvertently as they eat) with additional Fe. Therefore, if your horse is performing this behavior, it might be advisable to examine his diet more closely. 

Remember, the temptation of most horse owner’s is to over supplement their horses. However, this is often unwarranted, contributes to the expense of managing the horse, and may provide no benefits to the animal. Certainly overzealous supplementation may actually be harmful to the horse.

Key terms:Hypomagnesia – lowered blood levels of magnesiumHomeostasis –physiological ability to maintain an equilibrium through an interaction of complex relationships. For example, glucose homeostasis is an interaction between key organs (liver, kidney and pancreas) and the hormones (insulin, glucagon, cortisol etc)to keep glucose at a relatively constant levelNext month: We wrap up the left over minerals! Manganese, cobalt, chromium and iodine 

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