On feeding horses – 4

This post on feeding horses is entirely devoted to minerals. I would not say it is a neglected subject when it comes to horses’ diets, but there are a number of basic elements that are crucial to every living organism, including horses. Each of these elements is abundant in some types of food and scarce in others, so horses are deeply affected by what they eat. I will comment on some of the most important basic elements, but there are probably more that plays an important role, some of which we know little about.

Mineral is a collective name for elements that appear naturally in solid form at room temperature. Most of them are very toxic for living beings in large amounts, but in small amounts they are fundamental for life. Their relationship is complicated and they often affect each other in one way or another. The official recommendations of daily intakes are uncertain and even contradictory. The ways of analyzing and measuring samples are often quite imprecise. Still it is important for all horse owners to have some knowledge of the functions of different minerals and the symptoms of deficiency and toxicity, as well as their combined effects.

The absorption rate of most minerals is low, which means most of the minerals goes right thru the digestive system and ends up as fertilizer for the next generation of grass. For most minerals, all we have to make sure is that the minerals are present in the feed within fairly generous tolerances; the horse’s digestive system will then absorb the necessary amounts. There are theories that horses (and of course other animals as well) can sense if they have a deficiency and seek up feed that is rich in that particular mineral, whether it is a certain grass, herb or even dirt or rock. (Icelandic horses have been observed eating stranded fish and seaweed by the shores, at rare occasions.) Horses living in the wild with access to vast areas have the opportunity to graze many different plants on different soils, which is one reason they seems to get all the minerals they need. Domesticated horses are limited to eat what we humans provide. There is definitely a connection between the mineral content in the soil and in plants. Different species of plants, and parts of plants, contains different specific amounts of each mineral. The use of fertilizers and chalk will increase the content of some minerals. Some minerals are very abundant in the daily forage, but others may be scarce and consequently need more attention.

Now let’s jump to a presentation of the different minerals, what they are good for and some about how they relate to each other. When content in different feeds are mentioned I prefer to use g/kg feed (1 g/kg is equal to 0,1%) and some minerals are given in mg/kg feed (1mg/kg is equal to 0,0001% = 100ppm). All figures are given by “dry substance”(excluding the weight of water in feed). When it comes to RDI (recommended daily intake) I use mg/kg bw or μg/kg bw, where kg bw means body weight of the horse in kg. (1kg = 2,2046… lbs). Hope you followed me there, it is confusing and it takes some pondering. One begins to understand why my wife, being an emergency nurse, keeps telling me and the kids to always check things twice, at the least. It can easily be one order of magnitude wrong and that is disaster.

For each mineral I also give the amount of mineral content that corresponds to a typical hay ration (dry substance) of 1,5% of the horse’s bw, just to illustrate how much is provided thru a typical basic grass feed. (e.g. 15 lbs dry substance a day for a 1000 lbs horse, which is about 15/0,84= 18 lbs regular grass hay with a 84% dry substance.) It is appropriate to mention that hay, or any kind of forage can indeed be very different in mineral content for an almost endless number of reasons, but with the extremes ruled out, the figures given are fairly equivalent to “typical” Swedish grass forage.

Calcium (Ca)

 Mostly about bones. Nokota horses are known for having a strong bone structure compared to other comparable breeds and it is particular important to keep track of the Calcium intake regarding young growing horses. Calcium is involved in various enzymes and is vital for the cell membranes. Calcium also works together with Potassium to make the muscles work. Like many other minerals the horse, thanks to a natural hormone, regulates automatically how much Calcium that needs to be absorbed by the body, so as long as the Calcium/Phosphorus ratio is within limits there is virtually no risk of getting too much Calcium. It used to be believed that too high calcium intake would cause Osteochondrosis, but it has been proven wrong. One problematic mechanism about calcium is that it competes with Phosphorus (as it is absorbed in the same part of the small intestine), so if the Phosphorus intake is to large, calcium will not be absorbed in the right amounts, but instead taken from the reserves, meaning the bones. The Calcium/Phosphorus ratio should be 1:1 to 2.5:1 meaning; ideally twice as much Calcium as phosphorus and never ever less Calcium than Phosphorus, young growing horses can be given up to 6 times more calcium than Phosphorus on condition that the phosphorus need is fulfilled. To complicate things even more; if the Magnesium content in the food is high, it will help increase the Calcium absorption. Calcium/Magnesium by weight, should be over 2,7. The worst danger for Calcium absorption is oxalate (or oxalic acid), which even in very small amounts will ruin the Calcium absorption drastically! Some plants high in Oxalate are; buckwheat, sorrels, rhubarb, parsley, spinach, leaves of tea plant and most berries. If the Calcium intake is low, or if the Calcium/Phosphorus rate is wrong; the bone structure might be dangerously weakened by Calcium deficiency and for young growing horses it can cause Osteopenia and Metabolic bone disease. The common cause of ruining the Ca/P rate is when horses are given high rations of grains, not the quality of the hay.

Grasses typically contains about 6 g/kg by dry substance (varies between 2-14 g/kg). Legumes (clover and alfalfa) normally contain more, up to 12 g/kg. Beet pulp contains 6-8 g/kg and soybean 4 g/kg. Grains are low in calcium; oats and rice 0,5-0,9 g/kg, barley 0,6 g/kg and corn 0,4-0,5 g/kg, by dry substance.

RDI: 40 mg/kg bw

Grass hay 1,5% bw provides: 90 mg/kg bw

Chlorine (Cl)

Not altogether toxic. Chlorine or actually Chloride as the negatively charged atom is called has important functions in many ways; it is part of the fluid between the cells and controls the PH and the osmotic pressure (the difference of pressure between the inside and the outside of cells). Chlorine is also vital for the digestion and especially for the ability to digest fat. Lack of Chlorine can cause a reduced milk production among lactating mares, a weak appetite, general weakness and weight loss. To maintain the water/salt balance it is of course important to have free access of water.

The horse gets Chlorine from regular salt (NaCl) containing 610 g Chlorine/kg. Other sources are; grass hay 9 g/kg, legume hay 6 g/kg, beet pulp 1,7 g/kg and corn 0,5 g/kg.

RDI: 80 mg/kg bw

Grass hay 1,5% bw provides: 135 mg/kg bw

Magnesium (Mg)

For a cool strong horse. Most of the Magnesium is found in the bones, but most importantly: Magnesium is absolutely vital for muscle function. It is also an important component in the blood and in various enzymes. Magnesium absorption is a very stable process and no other minerals or substances have any severe effects on Magnesium absorption, so a good intake of magnesium can at some degree compensate for calcium, if you follow me. Calcium/Magnesium by weight, should be over 2,7. A horse typically absorbs half of the magnesium contained in the food and too much magnesium is no reason for concern. (It can be overdosed when Magnesium sulfate is given to solve a blocked intestine and then cause kidney problems) Magnesium deficiency on the other hand, leads to nervousness, bad coordination and shaking muscles. It may also cause clots in the aorta.

Forage contains on average 1,8 g/kg dry substance (varies between 0,7-3,5 g/kg) Oats contain about 0,15-0,2 g/kg. Magnesium can be given as a supplement in the form of magnesium-oxide, often given to cattle.

RDI: 15 mg/kg bw

Grass hay 1,5% bw provides: 27 mg/kg bw

Phosphorus (P)

Essential for life. Phosphorus is responsible for the energy production in the cells and it is also important for the cell membranes. Besides, Phosphorus is, just like Calcium, a main ingredient in the bones. Other functions involve nucleic acids (DNA) and Phospho-proteins. The absorption of Phosphorus is actually controlled by the horse’s body, but it is complicated; it depends on the Calcium content in the diet, the horses’ age, the type of Phosphorus source and even the outdoor temperature (low temperatures means less absorption), so it is climate dependent! The typical absorption is 35% and up to 50% for young horses and lactating mares. High Calcium levels limit the Phosphorus absorption, so again I must refer to the importance of the Calcium/Phosphorus ratio. Additional Sodium-Chloride (salt) will increase the Phosphorus absorption. Some of the Phosphorus present in plants is in the form of Phytate-Phosphorus, which is hard to absorb by the horse. Young horses and lactating mares are often given supplements containing inorganic Phosphorus which is easily absorbed (which is why their absorption rates are presumed higher). Phosphorus deficiency may cause softening of the bones (osteomalacia) and a problem similar to vitamin D deficiency.

Forage contains on average 3 mg/kg (varies between 1,5 – 6,6 mg/kg) Phosphorus, by dry substance. Oat, barley, corn grains contains 3-4 mg/kg. Wheat and rice bran 11-16 mg/kg, beet pulp 0,9 mg/kg

RDI: 28 mg/kg bw

Grass hay 1,5% bw provides: 45 mg/kg bw

Potassium (K)

Controls everything. Potassium is the main PH regulator in the horse’s body, by being the main positively charged ion (cat ion) inside the cells and it also controls the amount of water inside the cells (the osmotic balance). Most of the potassium is however involved in skeletal muscle excitability. The Potassium ion channels are positioned in the cell membranes and controls the flow of Potassium in/out of the cells. Potassium is actually responsible for making muscle cells move, which means it performs every movement, including the heartbeat. These ion channels also regulate the release of hormones, including insulin. Swedish soils are high in Potassium and the forage usually contains much more than needed. Deficiency is no concern, since the horse kidney is known to handle excess potassium very easily thru urine as long as water is available. If a horse is dehydrated she in fact refuses to eat food with excess potassium.

Forage is the main source of Potassium containing on average 20 mg/kg (varies between 6,5-43 mg/kg) by dry substance, while grains contain less than 4 mg/kg.

RDI: 50 mg/kg bw

Grass hay 1,5% bw provides: 300 mg/kg bw

Sodium (Na)

Cellular communications. Sodium has many important roles to play. Sodium assists the transport of signals in the nervous system thru excitable cells. Signals move thru a neuron by an action potential; positive charges travelling from cell to cell as an impulse is created when sodium is pressed into the axon (an outgrowth of the cell) and potassium is pressed out of the axon. Sodium transports all kinds of substances soluble in water, like amino acids and glucose, thru the cell membranes. Sodium is a very important electrolyte that controls the PH of cells and it also regulates the osmotic pressure. Because Sodium attracts water it controls the distribution of water thru out the body, in the kidneys, the digestive system and the cellular fluid. Most horse feeds contain very little sodium so usually we supplement by providing salt (NaCl) in the form of a salt block. However, horses have much softer tongues than cattle, horses may not get enough salt. Another way is to provide salt in loose form in a bucket. Horses seem to have a natural way to control the need of sodium. Sodium deficiency affects the skin, decreases the water intake, leads to loss of appetite, eventually it will affect the muscle control. Too much Sodium is no concern since it will be emitted thru urine, as long as water is available.

The horse’s main source of Sodium is regular salt (NaCl) containing 390 g Sodium/kg. The Sodium content in hay is scarse and varies between 0,1 and 0,25 g/kg dry substance.

RDI: 17 mg/kg bw

Grass hay 1,5% bw povides: 3 mg/kg bw

Sulfur (S)

A versatile building block. Some essential Sulfur substances are; Biotin which is important for the hooves, chondroitin for the joints, insulin which regulates metabolism of carbohydrates and finally methionine an essential amino acid. Cystine and methionine contains Sulfur and they are deeply involved in the structure of most proteins. Sulfur is also present in most enzymes. Thiamin involved in the carbohydrate metabolism contains sulfur. Heparin which is an anticoagulant (prevents clots in the blood vessels) contains sulfur. Sulfur is provided thru plants in the form of protein. Sulfur deficiency is rarely a problem, nor is toxicity.

Forage contains on average 1,2 g/kg dry substance. Soybean meal is a good source.

RDI: 23 mg/kg bw

Grass hay 1,5% bw provides: 18 mg/kg bw

Cobalt (Co)

For the blood. Cobalt is important for the formation of blood cells. The microbes in the digestive system are totally reliable of Cobalt to produce vitamin B12, which is combined with iron and copper to build blood cells. Deficiency is generally no concern, even if some rare soils are known to be deficient. Toxicity is unlikely since the absorption rate is very low.

Grass hay contains on average 0,12 mg/kg dry substance, but varies between 0,06 and 0,2 mg/kg.

RDI: 0,4 μg/kg bw

Grass hay 1,5% bw provides: 1,8 μg/kg bw

Copper (Cu)

A key element. Copper is the base for elastic structures as connective tissue (skin, ligaments, tendons, joints, hooves, skeleton…). Young growing horses need copper for bone collagen. Copper is also present in melanin and in blood cells. It also transports iron to where it is needed. Copper plays an important role in the central nervous system. The problem with Copper is that its absorption is sensitive to the presence of other minerals; calcium salts, ferrous sulfide, mercury, molybdenum, cadmium and especially zinc all reduces the Copper absorption. The Copper content in feed differs considerably and seems to be highly related to the content of Copper in the soil. Deficiency may cause bone and hoof problems. Toxicity is defined as feed containing 250 mg/kg.

Cane molasses contains up to 70 mg/kg. Hay and beet pulp usually around 10 mg/kg and oats somewhat less.

RDI: 150 μg/kg bw

Grass hay 1,5% bw provides: 150 μg/kg bw

Iodine (I)

For the metabolism. Iodine is important for the creation of the hormones thyroxine and triiodothyronine, which regulates the basal metabolism, it’s all it does and it’s absolutely vital. The concentration of iodine in feed is dependent on the concentration in the soil where the feed is grown. Iodine is usually supplemented only by iodized salts, which makes it a vulnerable source and puts more attention to the importance of mineral- and salt bocks. Toxicity may occur if horses are over-supplemented! The danger is that both deficiency and toxicity show the same symptoms; goiter (enlarged thyroid). If the urine is analyzed it is possible to get a hint whether it is an excess or a deficiency. A mare’s iodine deficiency or excess may also cause severe problems to their foals. Excess may cause reduced defense against infections.

Iodine content in forage and grain varies between 0,03 and 0,52 mg/kg but on average 0,24 mg/kg dry substance. (Seaweed like Kelp can contain up to 1850 mg/kg dry substance).

RDI: 1,75 μg/kg bw

Grass hay 1,5% bw provides: 3,6 μg/kg bw

Iron (Fe)

Oxygen transporter. Iron is responsible for the transport of oxygen in the blood since it is the base of hemoglobin. Myoglobin which provides the muscles with oxygen has iron as an important ingredient. Small amounts are also present in the liver, spleen, muscles, other tissues and various enzymes. The absorption rate is only around 15%, even less regarding older horses. On top of that; excess consumption of cadmium, cobalt, copper, manganese and zink will decrease the absorption. Deficiency may cause anemia, but it is very unlikely if horses have access to plain soil. Excess iron will decrease the zinc level in the blood and in the liver but it is usually no concern. Toxicity is difficult to detect, so large supplement of iron is not recommended and definitely not to young foals!!

The concentration in forage is typically 130 mg/kg dry substance, but varies between 40-250 mg/kg, grains contains less than 100 mg/kg dry substance.

RDI: 0,60 mg/kg bw

Grass hay 1,5% bw provides: 1, 95 mg/kg bw

Manganese (Mn)

For bones and carbs. Manganese is vital for the bones as an important ingredient in chondroitin sulfate. Manganese is important for the digestion of lipids and carbohydrates, it also activates several enzymes. It is a fairly rare mineral in the body and does not accumulate easily. The absorption rate seems to be difficult to determine, so the need of this mineral is hard to specify. Manganese is probably the least toxic of all minerals. Excess amounts can however reduce the phosphorus absorption, so the maximum intake of manganese is set to 400 mg/kg feed. Deficiency is rare, but may result in bone abnormalities.

Forage can range between 45 and 120 mg/kg dry matter (an average figure is 70 mg/kg), grains 15-45 and corn 10 mg/kg.

RDI: 0,60 mg/kg bw

Grass hay 1,5% bw provides: 1,05 mg/kg bw

Selenium (Se)

Rare but crucial. Selenium is important by many reasons. It fights heroically against toxic substances like oxidants that threaten the cell membranes. It also partly controls thyroid hormone metabolism. Generally deficiency is much more common than toxicity. Deficiency causes muscle weakness, which means problems with all kinds of movements, including respiration and heart problems. Toxicity may cause blind staggers (blindness, sweating, colic, lethargy, high heart- and breathing rates). It may also cause alkali (hair loss at mane and tail, cracking at the coronary band of the hooves). Large levels of selenium may also replace sulfur in some tissues like keratin (hooves and hair) meaning poor coat, weak hooves or even laminitis. A mineral block is one important selenium source. The concentration of selenium in plants is reflected by the concentration found in the soil. It is also dependent on the PH, since alkaline soils seem to encourage selenium absorption in plants. Plants with deeper roots tend to accumulate more Selenium, since Selenium tends to be more present deep in the ground, yet there seems to be no confirmed difference between alfalfa, clover and grass.

Grass hay may contain from 0 to 1,2 mg/kg dry substance, on average 0,1 mg/kg.

RDI: 1,75 μg/kg bw

Grass hay 1,5% bw provides: 1,5 μg/kg bw

Zinc (Zn)

Everywhere and everything. Zinc is a common ingredient in many important enzymes like carboxypeptidases, which runs the insulin production, wound healing and blood clotting. Zink is also important for the eyes, but is also present in muscles, bones and the skin. Severe Zinc deficiency may cause hair loss, poor appetite, and low enzyme production. Toxicity is normally no concern, the limit is 500 mg/kg feed. However zinc and copper compete in a similar way as calcium/phosphorus. There is no fixed rate, but higher zinc to copper is what may cause problems.

Beet pulp, grass and legumes contain 22-35 mg/kg, oats 40mg/kg, corn 25 mg/kg, rice bran 70 mg/kg and wheat middlings 90 mg/kg, dry substance.

RDI: 0,60 mg/kg bw

Grass hay 1,5% bw provides: 0,45 mg/kg bw

Any general recommendations?

I have tried to figure out some kind of recommended amounts of daily intake and to be frank there is some variation between references. It is not guaranteed that the need of all minerals is direct proportional to the amount of energy+proteins needed. It is even probable that the need of some minerals are independent of the rate of activity and that some minerals depend primarily of the horses age and gender, not the total daily feed intake. Other factors to consider are; which horse breed, which season or climate, etc. A common generalization is to make a split between horses and ponies, meaning that ponies are easy feeders, but is the height of the withers a good clue? For example; calcium is an important building block for skeleton bones and some breeds, like the Nokota Horse, have a heavier bone structure than others in the same size and weight, so shouldn’t their need of calcium be emphasized? (Another way to reason would be that breeds with lighter bone structures would be more sensitive against calcium deficiency since their reserves are smaller; a deficiency would take more calcium of the bones, by percentage.)

Some breeds, like the Nokota Horse, are adapted to an extreme inland climate with ice cold winters and hot dry summers with a feed supply that varies between seasons; their need of minerals might differ considerably between seasons. Pregnant and lactating mares, stallions, young foals and old horses all have different needs also when it comes to minerals. The soils where horses graze and where their hay is grown varies in mineral content, different plants have different mineral content and there are variations from one year to another. Do you see where this is leading? Right, it is much too difficult to precisely decide the need of minerals for a specific horse on a specific day. But given the circumstances, these are the conclusions:

Most minerals are sufficiently provided thru decent hay and a fair pasture to graze, but the following minerals need some attention; Ca (in relation to P and Mg), Cu, Zn, S and Se. Supplementation by mineral- and salt blocks as well as a supplement feed can fill in the gaps. If you have doubts about your hay, analyze it, but don’t overrate the results, keep a wide tolerance in mind. Make sure the ratios between Ca/P/Mg are well within limits. Provide lots of fresh water, a good mineral block and a salt block, check on the consumption rate and calculate.


11 thoughts on “On feeding horses – 4

  1. Geez! It was like reading a college textbook punctuated with great photos of the horses. (No, your daughter could never be mistaken for a horse so tell her not to be offended! )
    Great work my friend, while mineral amounts do vary depending on breed, I really thought your description of each minerals function and where it can be found was awesome. Also the warnings etc was good. Great read and I copied and pasted it into my horse care folder.


      • I was wondering about that.. One looked so much younger. You have beautiful children. (Not bad lookin’ horses either! LOL ) I enjoy seeing your place, you really take pride in it. If ever I am so lucky as to travel, your place is one I’d visit in a heart beat. 🙂


  2. i definitely learned some things here, too, thank you 🙂 So now a question: in your reading did you catch anything about the application of lime actually enhancing the deficiency of other minerals because the calcium bumped them from the soil colloids, much as many minerals can compete with each other when one is consumed in excess?

    It is always so interesting to read and ponder the science… but also very daunting since the more we learn the more we realize how much more we don’t know… so we have been buying a variety of different salt blocks and during the winter let the horses choose from several at a time. i believe that they are rather effective at balancing their own rations, and here in Ångermanland last winter when we fed mostly just grass hay with a good mixture of “weeds” (which are often relished and coincidence or not are also often mineral concentrators) plus a little oats at times was that our horses far prefer the wildlife salt block, which they devoured about 3 times faster than the horse, and during which time they barely touched the plain white block. We bought all of the salt blocks from Granngården, and i will list the horse first and wildlife second by each mineral: 0.11% v. 0.15% Magnesium, 400 mg/kg v. 2,000 mg/kg Coppar, 280 mg/kg v. 300 mg/kg Zinc, 200 mg/kg v. 200 mg/kg Manganese, 210 mg/kg v. 0 iron, 15 mg/kg v. 50 mg/kg Iodine, 15 mg/kg v. 60 mg/kg Cobalt, 5 mg/kg v. 30 mg/kg Selenium. So that was a very interesting, even if very preliminary experiment, and ever since the wildlife stone has remained a favorite, although at times they do consume the horse blocks nearly as quickly, but the white blocks sure get ignored.

    And your photos are absolutely beautiful, thank you 🙂


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