Written By Dr. Kris Hiney

Selenium is an often talked about micro mineral which has much confusion over its requirements. Certainly those in the nutrition field don’t make it any easier by listing the requirements in concentrations in the diet, where all other minerals are simply listed as the amount the animal should consume per day.

Selenium is an essential mineral that is integral to the enzyme glutathione peroxidase (GSH-PX). GSH Px is a powerful anti-oxidant which helps protect cell membranes, proteins and even DNA from reactive oxygen species such as peroxides, free radicals etc. The enzyme GSH-PX acts by donating an electron and thus reducing these reactive compounds. Less commonly known, Se also serves a role in thyroid metabolism. Selenium is a part of the enzyme thyroid hormone deiodenase, which serves to convert thyroxine (T4) to its more biologically active form, triiodothyronine (T3). Thus Se deficiency can play a secondary role in hypothyroidism.

Selenium is also a confusing mineral because it is more or less of a problem depending on the area of the country you live in. Areas of the country which typically have higher Se concentration in the soil include South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colorado, and New Mexico, while the Great Lakes region, the Northwest and the Southeast are considered Se deficient. The more alkaline or basic the soil, the higher the concentration of Se in the plants that grow there. Dry conditions will also encourage the plant to uptake more selenium, which further increases the Se concentration. High selenium concentrations in the soil can be detected by the abundance of indicator plants, which include locoweed, milk vetch, woody aster and false goldenweed. During drought conditions horses will also be more likely to consume plants they may not eat normally. Therefore it is important to closely monitor pastures for horses during dry conditions.

Se and your calculatorSo let’s talk numbers and get to the heart of the matter. The Food and Drug Administration regulates the amount of Se that can appear in swine, cattle and sheep feeds due to these animals typically entering the human food chain. For traditional livestock species, Se can only appear at a level of 0.3 mg/kg DM in complete feeds. That would be the concentration of the entire diet the animal consumes, not just the individual components. It is much more common in the livestock industry to feed complete rations or TMRs – total mixed rations. In horses, with the exception perhaps of complete feeds that many senior horses consume, we tend to feed forage with grains or some other type of supplement Even further, the total amount of Se beef cattle can consume per day is only 3 mg. This is according to Title 21, Part 573.920 in the Code of Federal Regulations. These regulations however, do not hold true of equine feeds.So what is actually the requirement of Se for the equine? Currently the recommendation to meet the horse’s nutritional requirements is to feed at 0.1 mg/kg or 1 mg/d. However, there is some evidence that feeding at rates of 3 mg/d may improve antibody status and overall immune function in the horse. No evidence exists that feeding at a rate of higher than 0.5 mg/kg in the total diet would be beneficial to the horse. Alternatively, there is also some pushback from environmentalists to reduce the level of Se in animal diets to only meet their requirement. They are encouraging the FDA to alter the current legal level of Se back to only 0.1mg/kg in the attempt to limit any Se accumulation in runoff etc. However, the contribution of Se from livestock feed is quite small compared to that produced through fuel combustion, industrial uses and leaching of selenificious rocks.

When we look at complete feeds for horses they typically contain between 0.3 – 0.5 mg/kg or ppm. A quick scan across commercially available equine feeds reveals a typical concentration of 0.3 mg/kg with some feeds slightly higher. In comparison, the Feed Additive Directive in the European Community allows Se to appear maximally in a concentration of 0.5 mg/kg. So how much Se would a normal horse on a complete feed consume? Let’s use a 500 kg horse for simplicities sake (that would be 1100 lbs). Typically we would assume the horse can eat 2% of its body weight per day. At an intake of 10 kg per day of a feed which contains 0.3 mg/kg, the horse would consume 3 mg of Se per day. So clearly horses can tolerate this rate of consumption quite well.

Signs of deficiency of Se in the horse general include disorders of the muscle or myopathies. This can include muscle weakness, gait abnormalities, respiratory distress and cardiac impairment. Foals born with a Se deficiency may have difficulty in nursing. Traditionally Se and/or Vit E deficiency disease is termed as white muscle disease. Numerically, serum Se less than 60 ng/ml or a GSH-Px concentration of less than 25 EU/dl can indicate a Se deficiency. However, these numbers are not indicative of a deficiency unless other clinical symptoms are present. Clearly much still remains to learn of Se metabolism.

Toxic Se?But what is considered a toxic level of Se and why are we so concerned with Se toxicities? The upper safe margin for horses is suggested to be at 2 mg/kg. That is essentially a single decimal point in difference when calculating rations. Now using our same horse, and assuming he still eats 10 kg, the horse has now consumed 20 mg of Se. This is a much narrower margin of safety than any other nutrient we include in the diet. Now remember, that is the total concentration in the diet, not of individual ingredients. Symptoms of acute Se toxicity include blindness, head pressing, sweating, colic, increased heart and respiration rate and lethargy. Chronic Se toxicity caused hair loss, especially of the mane and tail, and changes in the hooves leading to soreness, including cracking of the hoof below the coronary band. Most are familiar with the recent story of the polo ponies who all died after receiving a Se injection from the team veterinarian. Ironically, anecdotal evidence already existed that administration of injectable Se and vit E may cause anaphylactic shock. This is probably due to the carrier agent used and not the concentration of Se or Vit E.The form Se is in may also play a role in toxicities. Se that appears within amino acids (such as would be found in plants) is much better absorbed and thus may reach toxic levels more quickly. In plants, Se is found in the form of selenocysteine, selenocystine or selenomethionine. Inorganic sources of Se include sodium selenite and sodium selinate. While some studies have reported no difference in bioavailability, (essentially the rate at which a substance enters the circulation) others indicate that selenium from yeast sources results in a greater detected increase in tissues and blood. While there is evidence on both sides, many have moved to using an organic source of Se in feed. Thus diets that contain Se in the form of Se yeast can’t have more than 0.3 mg/kg of Se in the total diet.

Now let’s look at some feeds and determine their contribution of Se to the diet.

A loose mineral supplement that contains 35 ppm of Se and is fed at 2 oz. per day compared.
Or one which contains 15 ppm at 2 -3 oz per day.

First, we need to know that 1 oz is equivalent to 28. 3 grams. If we feed our horse 2 oz. per day, that would be equal to 56.6 grams of supplement. If Se is listed in ppm or mg/kg, we simply convert units. There are 1000 grams in a kilogram, so our horse is eating 56.6 g/1000 g/kg or 0.0566 kg of supplement. Now multiply that by our Se concentration. In our first supplement, 35 mg/kg * 0.0566 = 1.9 mg of Se. That is right in the middle between the Se requirements for the horse and the higher level of intake that has been shown to have beneficial effects. The supplement which contains 15 ppm would provide between .8 and 1.3 mg of Se depending on if you fed 2 or 3 oz of the feed.

So until next time, don’t panic about Se, now you know how to feed it correctly!

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Written By Dr. Kris Hiney

 

 

Last month we discussed the function of electrolytes and some special disorders of horses related to these minerals. This month we will look at how much of these minerals your feed usually supplies, and determine how much electrolytes you may need to try and supplement to your horse.

How much do they need?Overall, the diet of the horse should contain between 0.25-0.5% salt. So, for example, let’s assume we have a 500 kg horse that eats 2% of his body weight per day. In total, he would consume 10 kg of feed per day. If we use those ranges of intake listed above, he should be ingesting between 25 and 50 g of salt per day. Therefore, the minimum a maintenance horse should be ingesting is about 25 g of salt per day. This will provide sufficient sodium and chloride for the horse. If we look at some typical hays for horses, the sodium content is quite low. Let’s just pick one and do the math to see how it works out. If we use the generic cool season grass hay, feeding 10 kg of this hay per day would supply 8 grams of sodium, 213 g of K, and 92 g of Cl.

Table 1. Average values on a percent basis for sodium, potassium and chloride for four general types of hays.

Types of Hays	Sodium	Potassium	Chloride
Coastal grass	0.17	1.80	0.67
Cool season grass (mid maturity)	0.08	2.13	0.92
Legume (mid-maturity)	0.02	2.45	0.61
Mixed legume/grass	0.01	2.80	1.10

Table 2. Sodium, chloride and potassium requirements for different exercise classes of a 500 kg horse in grams/d.

	maintenance	light	moderate	heavy	intense
Na	10	13.9	17.8	25.5	41
K	25	28.5	32.0	39	53
Cl	40	46.6	53.3	66.5	93

Comparing this to the maintenance horses requirements listed in table 2, tells us that the horse really needs to only get extra sodium. In fact, only the heavily exercising horses will probably need additional chloride supplied to them. Again, this is heavily dependent on the temperature in which the horse is working.

What about grain?Many horses also receive a concentrate in addition to the forage they are eating. Typically, most horse feeds are formulated to contain between 0.5 and1% salt. This increased concentration of salt in the feed is based on the knowledge that most horses will be consuming less grain then hay. Let’s use our same horse above, and now feed him a diet that adds 5 lbs of grain per day. (Wondering how much hay and grain your horse should be eating? See Rules to Feed By) I’ll convert back to kg so that we can look at our numbers on a gram basis. 5 lbs of grain is equivalent to 2.2 kg. If we assume our grain has 0.5% salt in it, then it supplies 11 g of salt. That breaks down to about 4 g of Na and 7 g of Cl (salt is 39.3% Na). Therefore, your concentrate may be helping to meet your horse’s salt needs. However, the tricky part is that the salt concentration is typically not listed on the feed tag, so you really don’t know how much it is supplying (See Using Feed Tags). Therefore, to be safe, you should supply your horse with some sort of salt source in addition to his feed. If you look at most feeding guidelines for equine feeds this is why it is stated to also supply your horse with salt on a daily basis. Take for example Omega GRANDE. A one day serving for horses is 227 grams. For the horse to consume it’s minimum amount of salt, Omega GRANDE would have to be 11% salt if it was to serve as the sole salt provider! And that would be just for a maintenance horse. Horse will not consume feeds with high concentrations of salt, and salt addition can even be used in some livestock species to limit feed intake. Consider that many horses really don’t need to be eating grain to begin with, or at least a reduced amount to avoid obesity, supplementing salt is always a necessity.

Providing salt    

Since we know that typically feeds alone won’t meet our horse’s needs, (or we may not really know what they are supplying), the easiest way to meet the horse’s needs is to supply a salt block. Researchers have shown that on average horses willingly consume about 50 g from a salt block per day. However, the variability in intake is high. Individual horses may range between 9- 143 g of salt per day! Therefore, some horses will eat too much, while others not enough. Even the same horse may alter his intake of salt quite a bit from day to day. If you really like projects, and have a sensitive scale at home, you could determine your horse’s average salt intake per day (if he is kept alone with his block) by weighing it every day. Also, some horses just won’t eat their block. If your salt block shows no evidence of licking and is covered with dust, you have a non-licker. Alternatively, you could try to provide loose salt, which some horses prefer or specifically feed salt to your horse. So how much salt should you provide your horse per day, especially if he is a non-salt block licker? For your maintenance horse, that would be about 1 oz. which is 28 grams. If you prefer to use your teaspoons to measure instead, one teaspoon contains 6 g of salt. So your horse would need 4 teaspoons of salt per day.

Exercising horses    

A horse in heavy work requires about twice the maintenance amount, or about 50 grams of salt per day. However, for those intensely working in hot climates, some researchers have indicated their need for electrolytes may increase 9 fold. Now remember, these are probably the race horses, three day eventers etc. Obviously for the exercising horse in hot climates, they may not be able or willing to consume that much via their salt block, which is why it is important to look consider supplementing your horse. Now remember, these horses would probably be consuming more grain than our example horse above, due to the increased energy demands placed on them. Therefore, you may presume that they are taking in much more salt in the diet. If you are supplementing your horse with table salt, you would increase that amount from maintenance to 2 oz or 8 teaspoons (2 2/3 tablespoons), with an increase to 3 oz or 12 teaspoons (4 T.) in hot climates. There are also many commercially available electrolytes as well which can be added to water or provided in a paste form.

Getting the water back in Typically if you need to provide a horse with electrolytes, you should also be concerned with rehydrating the horse. Oddly enough, the horse’s own system can work against it. As the sweat of horses is so much more hypertonic (or contains more solutes) than its plasma, when horses sweat heavily, their blood becomes hypotonic. It does not provide the normal stimulus to drink that having a higher electrolyte concentration in the blood does. Therefore, even if offered water, your horse may not drink. Providing electrolyte pastes or saline solutions after exercise may cause the horse to restore his water balance and recover more quickly. However, do not just offer a horse a salt water solution if they have not been trained to drink it. This will result in water refusal and only exacerbate the problem. They should also be offered a choice of non-saline water to ensure that they replenish the water they have lost. In addition, horses seem to prefer tepid water to ice water when given a choice. So remember, it is as imperative that the horse is also restoring his water balance after exercise as it is to provide electrolytes.Next month we will look at two very important trace minerals, copper and zinc.    

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This month we will discuss two important trace minerals, copper (Cu) and zinc (Zn). We will discuss them together, as they are most commonly discussed in relation to developmental orthopedic diseases in young horses. First of all, copper and zinc are classified as trace minerals because they are required in far less quantities compared to Ca, P, Na, Cl etc. While the minerals we previously discussed were described in terms of percentages of the diet, or in grams, trace minerals are only required in mg per day. Typically their requirements are listed in ppm (or mg/kg) of the total diet. It is then assumed that a horse would be consuming a standard 2% of their body weight per day. Thus if a requirement is listed as 15 ppm, then a 500kg horse should consume 150 mg per day. Now let’s discuss what these particular trace minerals do for your horse.

Cu and its functionsCopper is a mineral that is heavily involved with collagen and elastin and their tissue integrity. Collagen is a structural protein found in skin, tendon, arteries, bone and cartilage, while elastin is found primarily in ligaments. Copper is necessary for a key enzyme (lysyl oxidase) which catalyzes the cross-linking of collagen, and thus is vital for the strength of cartilage, tendon etc. Essentially that is the connection between separate collagen fibers. That is why Cu is typically implicated in developmental issues in growing horses. Copper also plays a role in energy production as it is part of cytochrome c oxidase, an enzyme which transports electrons and helps to generate ATP (remember, that is the energy currency for cells) within the mitochondria. It is also necessary for iron (Fe) metabolism and serves in an anti-oxidant capacity. Copper easily accepts electrons and thus aids in scavenging free radicals. In fact, free Cu can actually cause free radical damage due to its ability to “grab” electrons. Thus in order to protect body tissues, Cu is found in blood bound to the protein ceruloplasmin. Both Cu and Zn are associated with the enzyme superoxide dismutase, a main player in preventing oxidative damage. The superoxide radical (oxygen with an extra electron) is one of the main reactive oxygen species (it steals electrons away from other molecules and makes them unstable) and can cause tremendous damage to the body unless it is eliminated. Superoxide dismutase catalyzes the reaction of two superoxide radicals to create hydrogen peroxide and oxygen. Finally Cu is involved in the enzyme tyrosinase, which catalyzes the production of melanin. Without melanin, hair loses pigmentation and can create funny colored animals!

Figure 1. Collagen is made up of several types of proteins. These proteins are wrapped around each other in a helical structure. The crosslinks between separate collagen molecules help to provide the stability in equine tissues such as cartilage, tendons etc.

Are horses different than other animals?Copper metabolism in horses differs from other species, so direct extrapolation from studies in other species may offer incorrect assumptions. For example, ruminants can suffer Cu deficiencies if fed a diet high in molybedenum. However, this does not happen in horses. In other species, alterations in hair coat due to Cu deficiency also appear more rapidly. In horses, only anecdotal evidence (never reported in trials where Cu deficiencies are achieved deliberately) supports the idea that the horse’s hair coat will differ (most often reported in black horses). And remember, sun can bleach hair coats and create a dull, reddish tinge too! Horses don’t show as extreme sensitivity to Cu overloads as well (sheep are especially susceptible) and can tolerate concentrations in the total diet as high as 250 ppm. Therefore, information concerning Cu and its metabolism, and requirements, needs to originate from equine trials.

Zn and its functions. Zinc is a ubiquitous mineral involved in over 100 enzymatic actions in the body. Key functions include digestive enzymes, bone function, and immune function, as well as the previously mentioned role as an anti-oxidant. Ironically, Zn may actually prevent free radical formation caused by other metals (primarily Cu and Fe which are highly reactive). When Zn is incorporated into proteins vs other metals, it may decrease the overall generation of free radicals. In humans, marginal zinc deficiency results in impaired taste and smell, as well as memory loss and decreased male fertility. In its anti-oxidant capacity, Zn helps to maintain cell membrane integrity, which may contribute to its immune function as well. Zinc deficiencies in animals and humans results in decreased resistance to pathogens (viral, fungal and bacterial) and even parasites and decreased antibody formation. The immunologic function of Zn is why it is commonly available in cold lozenges for people (it was clinically proven to decrease duration of the common cold). Currently no such studies have been conducted in the equine.

The role of Cu and Zn in bone disorders. Deficiencies in Cu especially have been implicated in developmental orthopedic diseases in young horses. This term can encompass such disorders such as physitis (inflammation of the growth plate), flexural limb deformities, wobblers (compression of the spinal cord in the vertebrae) and most commonly osteochondrosis or OCDs. It is also commonly referred to osteochondrosis dissecans once a flap of cartilage is free floating in the joint. Osteochondrosis is typically seen as a lesion in the cartilage due to improper ossification of the subchondral bone (bone underlying the cartilage in a joint). Management of young horses including diet and exercise, rapid growth rate, as well as genetics have been implicated as causes of these disorders. Experimentally, cartilage lesions and gait abnormalities were induced in foals fed a diet deficient in Cu, but these foals were also deficient in Ca and P. Thus the absolute cause is difficult to tease out. Unfortunately subsequent studies in foal development and Cu intake have had contradictory results. Overall the most benefits in Cu supplementation in preventing OCDs may be realized by supplementing the dam in late gestation. A Cu deficiency may not result in formation of OCDs (genetics, growth rate, other nutritional factors may actually cause the lesion development) but Cu may help in the repair mechanism. Visually, it may be difficult to determine if foals are deficient in Cu as their growth rate and outward appearance is normal. On a practical note, while Cu concentrations have been increasing in commercial equine feeds, especially in feeds formulated for young horses, a decrease in OCD have not been seen generally in the equine population. Now certainly, there are many confounding factors in the pathogenesis of OCD, genetics included. In the equine industry, we are guilty of breeding horses despite them having soundness issues. Thus, we may be inadvertently perpetuating developmental diseases in our horses.

Figure 2. Alterations in the normal process of ossification can result in cartilage defects.

Cu and Zn requirements. Ideally mature horses should receive a total diet of 10 mg/kg or 0.2 mg/kg BW/d. Therefore a 1100 lb horse which consumes 2% of its body weight in feed per day would consume 100 mg of Cu per day. Most natural horse feeds contain 3-20 ppm of Cu. Increases in Cu requirements due to exercise are slight, and most will be accomplished simply by increasing intake to meet energy demands. Current recommendations for Zn are to be fed at a rate of 40 mg/kg DM. Thus depending on the feed source of the horse, supplementation becomes more necessary. Typically mares fed a normal diet should supply adequate Cu in the milk for foal growth as no additional increases in milk Cu concentration were seen with Cu supplementation to mares. However, as a precautionary measure an increase in Cu in the diet of broodmares may be warranted. Feeds designed to be fed for foals often contain 50 ppm of Cu or greater and between 150 to 220 ppm Zn. These values are well above the current NRC recommendations as more of a safety precaution. However, overzealous supplementation of trace minerals is not a wise idea. Excess Zn can impair Cu metabolism as they share a common transport mechanism. Typically this wouldn’t be an issue unless Zn concentrations are over 200 ppm in the total diet. This might occur with a misformulation of a diet, or if the horses are pastured need a metal refinery. It is recommended for Zn to not reach a concentration over 500 ppm. Extreme levels of Zn can cause joint abnormalities but only in excessive quantities (such as 2% of the diet).

Figure 3. The cartilage defect in the hock joint is circled in red.

Next month we will discuss Mg and Fe.

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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