Minerals for Horses: Calcium: The Building Block

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Written by Dr. Kris Hiney

This month we will return to discussions of nutrient requirement for horses. Remember we discussed energy needs for horses in the earlier articles: Equine Energy Requirements, Energy for Work, and Broodmares and Babies. Now we will look more closely at other nutrient requirements, beginning with mineral requirements. Minerals are involved in a variety of functions in the body, including enzymes, structural components, energy transfer and acid base balance. Minerals are also incorporated into vitamins, amino acids, and hormones. Thus proper mineral nutrition is vital to have a healthy horse. The minerals that are needed in the largest quantities by horses are referred to as the macro-minerals. These include calcium (Ca), phosphorous (P), magnesium (Mg), potassium (K), sodium (Na) and chloride (Cl). These minerals are needed in the diet in concentrations of g/kg or percentages, versus ppm or mg/kg of micro-minerals. Today we start with those most commonly talked about in equine nutrition, Ca and P.

A note of cautionIt is important to remember that when creating diets for horses, we consider not only how much mineral is in the diet, but also the ratio of particular minerals in relationship to others. Minerals have very complex interactions with each other, and excesses or deficiencies of minerals can greatly affect the absorption, metabolism and excretion of others. Therefore horse owners who “tinker” too much with their horse’s diet through overzealous supplementation may be doing more harm than good for their horse.

Previously in Broodmares and Babies we had mentioned that the ratio of Ca and P is always important when looking at horse rations. If you recall, we would like to see a ratio of 2:1 Ca:P, with a range of 1:1 to 6:1 being acceptable. What you should avoid is a total diet which is less than 1:1 or in other words, offers more P than Ca. This is due to the fact that phosphorous competes with Ca for absorption in the gut. Remember to calculate the entire diet however! And while the proper ratio is important, it is possible to have the correct ratio, but still be deficient in these minerals if they are in insufficient quantities in the feed. Now let’s talk in more detail about these very important minerals.

 

How important is calcium?
Commonly most people think of calcium’s role as that of bone development and integrity. Certainly the skeleton does account for 99% of the calcium in the horses’ body. However, Ca is absolutely essential for neuromuscular function, blood clotting, cell signaling, and an array of enzymes. Because of its importance, calcium concentrations are very tightly regulated in the blood. When Ca in the diet is inadequate, the bone serves as a major reservoir. Thus the body will sacrifice calcium within the bone to maintain blood Ca homeostasis. A prolonged period of Ca deprivation can lead to a weakened skeleton. In addition, this means that blood values of Ca are relatively poor indicators of Ca status in the horse. Calcium deficiencies are especially detrimental to young growing horses as this can lead to osteopenia. Improper ossification can lead to enlarged joints or improper growth patterns of the long bones. Therefore, it is critical to look carefully at the diet of lactating mares and foals.
Calcium requirements
Adult horses which aren’t exercising are relatively easy to meet their calcium requirements as can be seen in Table 1. The increase in Ca requirements for exercising horses is presumably due to an increase in bone deposition. Horses undergoing intense exercise experience an increase in bone mass and thus have a greater need for calcium. (More on bone formation in upcoming articles) It is unlikely that light exercise, or exercise that the horse is already adapted to (essentially no change in work intensity) results in much change in calcium requirements. Additionally most studies of calcium and exercise have focused on the young, growing horse. However, in an effort to err on the side of safety, the National Research Council recommends higher intakes of calcium. There is some loss of Ca in the sweat of exercising animals which is also represented in the increase in requirements for work.
Weight (lbs.)MaintenanceLight workModerate workHeavy work
90016242832
100018273136
110020293439
120021323843
130023354146

Table 1. Calcium requirements (in grams/day) for adult horses at maintenance or work. To determine which class your horse fits into, read Energy for Work.

 

Mommas
For gestating mares, requirements of Ca increase the greatest for the 9th, 10th and 11th month of gestation, which is concurrent with the most rapid increase in fetal growth. However, there is still substantial fetal growth in the 7th and 8th month of gestation as well, and so Ca requirements are greater than maintenance. Lactating mares clearly have an increase in Ca demand in order to support the very rapid growth of their foal. Mares fed an inadequate amount of Ca actually experience a decrease in bone density (as detected through radiographic analysis of the cannon bone). If you compare Table 1 with Table 2, you can see that at least in terms of Ca horses at light to moderate work would be considered to be comparable to gestating mares. However, demands of lactation far outstrip the working horses in needs of calcium. Therefore, one should either choose a feed for lactating mares and babies, or a supplement designed to meet their needs. After the first three months of peak lactation, the calcium demands on the mare taper off as the foal derives more of his nutrition from the feed he consumes.
Weight (lbs)Month of GestationMonth of Lactation
–6th7th-8th9th-11th1st2nd3rd4th
90016232948474534
100018253253535037
110020273558585541
120021303963636045
130023324269686548

Table 2. Calcium requirements (g/d) for gestating and pregnant mares. Remember to use Table 5 from Broodmares and Babies to determine how much your mare should weigh.

 

The Babies
Obviously foals get much of their Ca from their mothers’ milk, but as they start to ingest new feeds and taper off their reliance on mom, it becomes your job to balance their diet. In the following table, I list the approximate Ca requirements of growing foals from 4 months until 2 years of age. Now, in just looking at the table, one might think that the Ca requirements appear low, but remember they are listed in grams per day! Because the foal is much smaller, he eats much less per day, thus the concentration of Ca in his feed must be greater. For example, a foal which consumed 2% of its body weight in alfalfa hay that was 2% Ca would meet its requirements, but if it was eating orchard grass hay that was 0.4% Ca would definitely not! There is not a very large decrease in overall Ca requirements as the foal matures. But as the foal matures and reaches a larger body size, it will consume more and thus the concentration of Ca needed in the diet will go down. Confusing, right? Don’t worry, next month we will discuss P requirements and then more importantly, how to put this all together with calculations from what you might be actually feeding.
Estimated Mature Weight4 mo-7 mo8-14 mo15-24 mo
900313030
1000353433
1100383736
1200424140
1300454443

Table 3. Approximate Ca requirements (g/d) for growing foals based on their estimated mature weight.

How big will my baby be?

How do you know what size they will be? Look at both the mare and the sire, and use an average. However, foals from maiden mares and older mares, tend to be smaller. Don’t forget that the nutrition program and environment that the dam and sire were subjected to also played a large role in their final growth. Also, foals carried by recipient mares will have a large influence in their size due to their foster momma, and less of an extent by their genetic mother. That’s why most farms choose larger mares for embryo transfer programs to carry donor mare’s babies. In addition, the early a colt is gelded, the larger they may mature to be. All in all, there is no firm way to know, but we can use our best estimation!

How much is too much?
Calcium has been fed as high as five times the horse’s requirement without any ill effects provided that the P intake is adequate. The maximal concentration of Ca in the horse’s diet is 2%, however it would be hard to find feeds that reach that level. However, excess Ca has been implicated as a causative factor of ulcers (See Is My Horse Stressed Out) due to an increase in gastrin secretion. Alternatively, others have found that alfalfa diets (and thus higher Ca) may decrease the incidence of ulcers. Clearly more work regarding Ca and ulcer formation in the horse is needed.
Definitions:
Osteopenia – a decrease in bone mineral density below normal. In humans this is considered to be a precursor to osteoporosis. Horses don’t really suffer from osteoporosis.
Ossification – essentially proper bone formation replacing cartilage as the horse grows, not to be confused with calcification. While calcification is a normal process of ossification, abnormal calcification can also occur, for example the formation of splints.

Gastrin – a peptide hormone secreted by the parietal cells of the stomach which stimulates secretion of gastric acids (HCL) and increases gastric motility. Gastrin release is stimulated by the presence of protein in the stomach, as well as conditions of hypercalcemia.

 

Minerals for Horses: Managing CA&P for Horses

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Written By Dr. Kris Hiney

Last month we took a more in depth look at the calcium requirements for horses. This month we will look at phosphorous needs in the horse, and then put it all together in formulating some diets for horses.

 

What does phosphorous do?
Many horse owners know that P is important for bone growth and skeletal health in their horses. While Ca is the major player, P makes up 14-17% of the mineral component of a horses skeleton. But that is not all it does. Phosphorous is vital in energy transfers (a little molecule called ATP – the P stands for phosphate!), DNA and RNA synthesis, cell membranes etc. Your horse would be going no where without P! Therefore, we shouldn’t forget P when looking at our horse’s diet.
What do you need to know?
For many horse owners, P is usually not to close to the forefront of our minds as most diets for horses will be adequate. However, the same is not true if you raise other livestock species. Phosphorous runoff is an environmental concern, and not something that even horse owners should completely ignore. Researchers have shown that high calcium diets may lower the ability of the horse to absorb P. This does not hurt the horse, as they are still absorbing a sufficient amount to meet their needs. But when we decrease absorption from the gut, you increase excretion in the form of feces. Now horse manure has a lower proportion of water soluble P in it compared to other species, and thus is less of a threat of contributing to run-off, especially in a pasture management situation. This may not eliminate the need for concern for larger stables with more concentrated numbers of animals. It may not be too long until the horse industry also comes under the scrutiny of the EPA. Now don’t panic! The point is that we simply want to avoid the random supplementation of minerals to horses without full consideration of the diet.
Phosphorous deficiencies and excessPhosphorous deficiencies are typically not seen in mature horses, even when exercising. However, just like with Ca, special attention needs to be paid to the broodmares and babies, who are both busy forming new bone! Certainly inadequate P results in a slowing of the growth rate of young horses, and can lead to improper bone formation.
However, excess P can be more of a concern. As stated last month, excess P can inhibit Ca absorption, which is why we always check the Ca:P ratio. Prolonged conditions of excessive P can lead to the development of secondary nutritional hyperparathyroidism. So what is the limit? Horses should not be fed more than 1% of their diet as P (and this is assuming a correct ratio). Typically this is not an issue unless the feed source is predominantly cereal grains (like wheat bran or oats) which are high in P. Conceivably this could happen on all grass forage (ie not legumes) with a high grain intake containing no mineral supplement as a balancer.
RequirementsAs the tables below show, maintenance horses will be fairly easy to meet their P requirements. In exercising horses, most of the requirements were determined using young horses who were also concurrently laying down more bone. However, again, the P requirements for mature exercising horses are estimated to be higher, more as a margin of safety.
Gestating broodmares and lactating mares again have higher P requirements, which you can see in Table 5. And just like last month, while the babies requirements might look low, they are listed in grams needed per day. That means that overall they need to have a higher concentration of P in the diet. Babies that are being fed for rapid growth without properly balanced P levels in the diet can certainly lead them to develop joint disease.
Weight (lbs.)MaintenanceLight workModerate workHeavy work
90011141723
100012161926
110014182128
120015192331
130016212434

Table 4. Phosphorous requirements (in grams/day) for adult horses at maintenance or work. To determine which class your horse fits into, read ENERGY REQUIREMENTS FOR THE WORKING CLASS HORSE.

Weight (lbs)Month of GestationMonth of Lactation
–6th7th-8th9th-11th1st2nd3rd4th
90011162131312921
100012182334343223
110014202638373526
120015212841413928
130016233044444230

Table 5. Calcium requirements (g/d) for gestating and pregnant mares. Remember to use Table 5 from BROODMARES AND BABIES to determine how much your mare should weigh.

Estimated Mature Weight4 – 7 month8-14 month15-24 month
900171716
1000191918
1100212020
1200232222
1300252524

Table 6. Approximate P requirements (g/d) for growing foals based on their estimated mature weight.

What’s in a feed
Before we start formulating some diets, it might be helpful to look at some typical feed stuffs that we use to create diets for horses. The following values are estimates only, and remember that forages grown on mineral deficient soils may have lower values. Typically, most grains are going to be higher in P than Ca. This should not pose a problem as concentrates should be fed to provide the extra energy or protein the horse needs, rather than making up the majority of their diet. The exception in the table is beet pulp, which isn’t a cereal grain at all. When looking at some typical hays, we can see that legumes (the alfalfa and red clover) provide much greater amounts of Ca than do the grass hays. This makes them ideal choices for broodmares and babies. Even most grass hays have Ca in the correct proportion to P, which makes feeding an imbalanced ratio hard to do. Note however, that orchard grass may be this exception.
Concentrate% Ca% P
Beet Pulp.89.09
Barley.06.39
Cracked Corn.04.30
Rolled Oats.11.40
Rice Bran.071.78
Wheat Bran.131.18

Table 7. Common concentrates fed to horses. All % are on a dry matter basis.

Forage% Ca% P
Coastal Bermuda.19.27
Alfalfa1.27.24
Brome Grass.29.28
Red Clover1.38.24
Fescue.41.30
Orchard Grass.27.34
Timothy.48.23

Table 8. Common forages fed to horses. All hays are assumed to be harvested at the midbloom stage. Remember that soil type as well as stage of maturity can alter your hays nutritional content.

Now, let’s put some of these numbers together. For simplicities sake, we will work with a generic 1100 lb horse, and then compare the numbers we get with the table values for different classes of horses. For our first example, we will feed this horse two different diets, one solely Bermuda Coastal grass hay, and one of alfalfa. We will feed him at 2% of his body weight per day. All values above are in dry matter, so we won’t have to convert those values.

First, we determine how much the horse will eat.
1. 1100 lbs x .02 = 22 lbs
Now remember all of our Ca and P requirements are in grams, so lets’ convert lbs to kg.
2. 22 lb x 1 kg/2.24 lbs= 9.82 kg of hay
So we know our horse will eat 9.82 kg of our hay per day.

For the grass hay, we will multiply the amount fed by the percentages of Ca and P in that hay.
1. 9.82 kg x 0.0019 =0.0187 kg of Ca
For that to make sense, convert kg into grams.
2. 0.0187 * 1000 = 18.7 g Ca
Now for P.
3. 9.82 kg x 0.0027 x 1000 g/kg = 26.5 g P
Finally, calculate your Ca to P ratio.
4. 18.7/26.5 = 0.71 to 1

So, what does this tell us? First of all, we probably need to have a mineral supplement for our horses to avoid the inverted Ca to P ratio. Alternatively, we could add some legume hay to its diet. When we look at simply meeting the requirements of our 1100 lb horse, we can see we are deficient in Ca if it is a working horse and certainly very low if that was all we fed to a gestating or lactating mare.

Now, what if we fed alfalfa hay instead? Use the same calculations, but insert the new % of Ca and P for alfalfa.
1. 9.82 kg x 0.0127 x 1000 g/kg =124 g Ca
Now for P.
2. 9.82 kg x 0.0024 x 1000 g/kg = 23 g P
Finally, calculate your Ca to P ratio.
124/23 = 5.4:1

Now we can see that our Ca to P ratio is more desirable. Looking at the horses’ requirements, we can see we have more than met the Ca requirement for all classes of mature horses, and are adequate for P for all working horses except those in heavy work. For mares, we are good until the last part of gestation and through lactation. That shouldn’t surprise you, as broodmares should be fed a better quality diet than our other horses.

Lastly, let’s see what happens if we decide to add 6 lbs of oats to this 1100 lb horse’s diet. Note: I am doing this solely for the purpose of calculations. There should always be some rationalization for why we add concentrate to our horse’s diet. For my mythical generic horse in this example, we don’t even know what class it is in or what its body condition score is.

For this example, we will change hays and feed red clover and orchard grass hays.
Let’s begin with the red clover.
1. 9.82 kg x 0.0138 x 1000 g/kg =135 g Ca
Now for P.
2. 9.82 kg x 0.0024 x 1000 g/kg = 23 g P
We will now calculate the contribution from our oats.
3. 6 lbs x 1 kg/2.24 lbs = 2.7 kg oats
4. 2.7 kg x 0.0011 Ca x 1000 g/kg = 3 g Ca
5. 2.7 kg x 0.0040 P x 1000 g/kg = 11 g P
Add the two values together for hay and oats
6. 135 g Ca from hay + 3 g Ca from oats = 138 g Ca
7. 23 g P from hay + 11 g P = 34 g P
Calculate your ratio
8. 138/34 =4.1:1

Looks good!!!

Now again, compare across our class of horses. Calcium is adequate for all classes, and our P requirements are met for all horses except the lactating mares.

Lastly, we will try adding oats to our orchard grass hay. 1. 9.82 kg x 0.0027 x 1000 g/kg =26.5 g Ca
Now for P.
2. 9.82 kg x 0.0034 x 1000 g/kg = 33.4 g P
Our oat values will remain the same as above.
Add the two values together for hay and oats
3. 27 g Ca from hay + 3 g Ca from oats = 30 g Ca
4. 33 g P from hay + 11 g P = 44 g P
Calculate your ratio
30/44 =0.68:1

Doesn’t look like something we should do!!!

To wrap up, remember that these diets were a simple exercise in calculating the contribution of calcium and phosphorous from different feed sources. These are not recommendations for actual diets, as we made no attempt to adjust amount fed, supplements added, or appropriate concentrates selected. Next month we will add in a more sophisticated approach to balancing diets inconsideration of type of horse, energy requirements, and growing horses. Until then, have fun practicing with your calculators!

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Minerals for Horses: Ca and P: Putting It All Together

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Written By Dr. Kris Hiney

 

Greetings again fellow horse owners! Hopefully everyone has had a chance over the last few months to start thinking about mineral requirements for horses in mathematical terms. To review, we have discussed the importance of Ca and P, especially in consideration of broodmares and young, growing horses. We have worked on calculating the Ca to P ratio you are providing in your diet, as well as comparing the total amount fed to the animal’s requirements. The suggested daily intake of Ca and P has been provided in tabular form, as well as some common feedstuffs’ concentration of these two minerals. Most importantly, we have worked out the math step-by-step, in order to allow you to really take a close look at your feeding program. This month we will more directly address the idea of feeding young horses properly.

Let’s talk babies!Again, review the last two articles which provided the amount of Ca and P that is needed by young growing horses. Compared to maintenance horses and exercising horses, the amount they need appears relatively small. But remember, their body size is also small, as well as the amount they can consume in one day! I think it is worthwhile to spend some more time specifically on this important class of horses. One of the difficulties in determining your young horse’s requirements is that they are usually listed according to their mature weight. Certainly that is how I presented this material in the last two months. But in order to help you out, the following table provides information on how much your young horse should weigh at various ages. This, of course is still according to their expected mature weight, which is at best an educated guess (See Minerals for Horses: Calcium). It is always important to monitor your foal’s weight gain, and determine if they are gaining excessive body fat. In addition, observe their joints carefully. Swollen and perhaps painful joints may indicate too rapid of a growth rate. For long term soundness, this is important to avoid.Table 1. Expected weights of young horses predicted by their expected mature weight. Note: these values would be typical of foals gaining weight at a moderate rate of growth.

Mature Weight (lbs)4 month6 month8 month10 month12 month14 month16 month18 month20 month
900302392461524578623663697726
1000336432513582642692737775806
1100370475564641706764811853887
1200403571614699771831885930968
130043756066875783690095910041048
How much will Junior eat?Intake in young horses is varied, and certainly will be different due to feed availability, palatability of the feed, and competition between herd members. However, on average, young horses will eat between 2 and 3% of their body weight on a dry matter basis. So let’s assume we are feeding a 4 month old foal who will mature to 1100 lbs 2.5% of its body weight per day. We will feed him a mixed grass and legume hay that contain 1.09% Ca and 0.35% P. Now, let’s see if we are meeting his Ca and P requirements.The foal is currently 370 lbs. If he consumes 2.5% of his body weight that will be 9.25 lbs of hay per day. Let’s convert now to kg, remembering to divide by 2.24. Thus the foal is eating 4.1 kg of hay per day. Calculating by the percentage of Ca and P, this hay will provide 45 grams of Ca per day and 14 grams of P. Need a review? Revisit Part 2 in our series of minerals for horses. Now look at your tables. Our foal is meeting his Ca requirements but is 12 grams short of P. So what should we do? This is a situation where concentrate feeding is clearly warranted. So let’s select a suitable feed for our baby and recalculate.

We will adjust our feeding scheme to feeding our foal 1.5 % of his body wt as forage and 1% as concentrate. Feeding such a diet that s approximately 65% forage and 35% concentrate is fairly typical for young horses. Previously young horses were believed to need a much higher proportion of grains in the diet, but recent research has shed light on better strategies for feeding young horses. This diet will now provide 2.5 kg of hay and 1.7 kg of concentrate. The feed we select to use is designed for young horses and broodmares. It contains 1% Ca and 0.55% P. When we recalculate, this diet now provides 27 g of Ca and 9 g of P from the hay, and 17 g of Ca and 9 g of P from the concentrate. Added together, the foal is now receiving 44 g of Ca and 18 g of P. He is still a little bit shy in phosphorous, but I don’t want to add more grain to the diet. Alternatively, I could select a feed that provides slightly more P.

I decide to use a select a new feed which contains a higher percentage P at 0.7% while the Ca content remains the same. All figures would remain the same with the exception of the P contribution from the new feed is 12 g of P. We have reached our goal of feeding our new guy a balanced ration (at least in terms of Ca and P)!

Now, for the sake of argument, let’s try to formulate a new ration that relies on slightly less grain for our growing foal. We will feed 2 % of his body weight in hay, 0.5% in concentrate and add a mineral supplement. The mineral supplement I chose has a concentration of 13% Ca and 12% P. It is designed to be fed at the rate of 2-4 oz per day. We will feed our baby 2 oz of the supplement per day.

Hay : 3.3 kg/day (2% of his body wt)
3.3 kg x 1.09% = 36 g Ca
3.3 kg x .35% = 12 g p

Grain: 0.8 kg (0.5% of his body wt)
0.8 kg x 1% = 8 g Ca
0.8 kg x 0.55% = 4.4 g P

Mineral supplement: 2 oz per day
Note: There are 28.35 g per oz. Therefore this colt will receive 56.7 g of supplement
56.7 g x 13% = 7.4 g Ca
56.7 g x 12% = 6.8 g P

All together the diet now provides 51 g of Ca and 23 g of P. This meets both his Ca and P requirements while allowing you to feed a more forage based diet. Now, which should you do? Well, that’s a topic for an entirely new discussion!

Next month we will continue on with minerals, but discuss one of the most confusing, miscalculated minerals there is – selenium. Until then, happy horse feeding!

 

Minerals for Horses: Demystifying Selenium

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 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!

 

Minerals for Horses: Supplying Electrolytes To Your Horse

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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 HaysSodiumPotassiumChloride
Coastal grass0.171.800.67
Cool season grass (mid maturity)0.082.130.92
Legume (mid-maturity)0.022.450.61
Mixed legume/grass0.012.801.10

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

maintenancelightmoderateheavyintense
Na1013.917.825.541
K2528.532.03953
Cl4046.653.366.593

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 inTypically 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.

 

Minerals for Horses: Copper and Zinc

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 Written By Dr. Kris Hiney

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 functionsZinc 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 disordersDeficiencies 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 requirementsIdeally 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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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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Minerals for Horses: Sulfer, Manganese, Cobalt & Chromium

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Written By Dr. Kris Hiney

This month we wrap up our discussion of minerals required by horses with a random mix of macro and micro-minerals. We will focus on sulfur, manganese, cobalt and chromium. What these minerals share in common is that they are typically in adequate supply in the average horse’s diet. However, they still warrant attention and understanding as they are vital for the health of the horse. Next month, we will start a new series on protein.

Sulfur is technically a macro-mineral, as it is required in larger amounts in the diet. Sulfur is most commonly found incorporated into two amino acids, cysteine and methionine. In the body, sulfur would be found in proteins (as cysteine and methionine) as well as some key vitamins: thiamin, biotin, and lipoic acid. Proteins containing sulfur include most structural proteins, as well as insulin, heparin and key enzymes: gluthathione peroxidase (an anti-oxidant enzyme) and as part of coenzyme A (an essential part of energy metabolism). Horses ingest sulfur in the diet mainly through consumption of protein. Horses receiving adequate protein that is of sufficiently high quality typically do not need additional sulfur in the diet. The total amount of sulfur in the diet should be about 0.15% on a dry matter basis. Therefore, unless the horse is suffering from protein malnutrition, or is deficient in these specific amino acids (most likely methionine), the horse should be adequate in sulfur. To this date, no one has been able to confirm a sulfur deficiency in horses provided that they are not deficient in protein. Some inorganic sulfur (in the form of a sulfur salt) can be incorporated into chondroitin sulfate (an important component of joint health) (Figure 1) and insulin in the body. More benefits are seen when feeding inorganic sulfur to ruminant animals, due to the ability of the rumen microbes to synthesize the sulfur-containing amino acids de novo (or from scratch). While this can indeed occur in the horse, the site of microbial synthesis is past the primary location of protein digestion and absorption and therefore does not yield the same benefits as seen in the ruminant animal. So bottom line for sulfur, if you are not deficient in cysteine and methionine, you are likely to be adequate in sulfur.

 

Figure 1. 

Structure of chondroitin sulfate. Chondroitin sulfate is a disaccharide unit of alternating monosaccharides (glucuronic acid and N-acetylgalactosamine) that is repeated in a non-branched form. This chain of glycosyl units are attached via a protein t

Cobalt

Cobalt is a trace mineral which is found in the body solely as cobalamin or vitamin B 12. Cobalamin’s function, and thus cobalt’s, is rather exclusive, limited strictly to serving as a cofactor for only two enzymes. While only two, they are vital for RNA and DNA synthesis and integral in carbohydrate and lipid metabolism. Those familiar with human nutrition know that cobalamin is found only in animal products, and thus can be deficient in vegans unless they supplement their diet. So how would a horse, who clearly is an herbivore, meet their cobalamin requirement? Bacteria in the hindgut of the horse are able to incorporate cobalt into the complex structure of cobalamin (See Figure 2). Vitamin B12 is then absorbed from the gut of the horse, thus supplying their needs. Currently no cobalt or B12 deficiencies have been reported or induced in horses. They actually seem to be more efficient at extracting cobalt from their environment or conserving cobalamin in their tissues than are ruminants. Horses have remained healthy on pastures that induced deficiency symptoms in cattle. Suggested toxicity limits for cobalt are 25 ppm in the diet with recommended dietary intakes set at 0.5 ppm.

 

Figure 2. 

Cobalt appears in red in the center of the ring structure.

Manganese is a trace mineral involved in both carbohydrate and lipid metabolism and is needed for the synthesis of chondroitin sulfate, whose structure is seen above in Figure 1. Manganese is also part of two key gluconeogenic enzymes as well as arginase, an enzyme in the urea cycle. It also functions as an anti-oxidant incorporated into a form of super-oxide dismutase. Remember that these enzymes function to eliminate the superoxide free radical, converting it to hydrogen peroxide in the body. Hydrogen peroxide is further reduced by other anti-oxidant enzymes in the body. Manganese has even been researched as its potential use an anti-oxidant drug for horses through infusions of inorganic manganese salts.

Most commonly manganese deficiencies are reported to impair cartilage metabolism and may result in growth abnormalities such as contracted tendons, epiphysitis etc. This is due to its role in the synthesis of chondroitin sulfate and thus in articular cartilage In Figure 3. You can see the bottle brush appearance of the proteoglycans attached to hylaronic acid. This provides the resistance, or cushioning effect of cartilage during use of the joints. When cartilage is not allowed to form properly, abnormal growth usually results.

The current recommendation for manganese is 40 ppm in the diet, with typical horse feeds ranging between 15-140 mg/kg. Forages are typically higher in manganese (40-140 ppm) than concentrates. Therefore, the average horse should be adequate in manganese. The source or form in which manganese is fed does not seem to matter. In the current literature, there does not appear to be any difference in bioavailability between organic and inorganic sources of manganese. Yearling horses fed below the recommended value did exhibit a slower growth rate compared to horses fed a diet at twice the recommended intake of manganese. However, in this example it would be difficult to determine if a faster growth rate was achieved due to managnese supplementation, or a slower growth rate resulted from manganese deficiency. Manganese toxicity is a concern in many species, and excess manganese can interfere with P absorption. The safe level of intake of manganese in the horse is suggested to be at 400 ppm. While never researched in horses, in humans Fe supplementation can inhibit manganese absorption (another reason to not over-supplement minerals!)

 

Figure 3 A.

Figure 3 a and b. Incorporation of chondroitin sulfates into proteoglycogen.
Figure 3 B. 

Chromium

Finally, chromium is an important mineral for both carbohydrate and lipid metabolism. Chromium enhances or prolongs the time that insulin is bound to its receptor, thus amplifying its effects. The exact mechanism by which it does so is not yet known. Thus it has created much interest in those individuals looking to enhance insulin sensitivity both in humans and in horses. In humans with insulin resistance or type II diabetes, supplemental chromium has been reported to improve glucose dynamics, but the results have not been consistent. In general, the majority of human studies have yielded very mixed results, with more benefits in humans with altered glucose metabolism, but not in diabetics. In addition, insulin resistance increases chromium excretion, which may be leading to a chromium deficiency. As of yet, chromium supplementation in a population of insulin resistant horses has not been performed. However, there has been some equine work that suggests a positive benefit to supplementation. Exercising horses supplemented with 5 mg of chromium from a yeast source had lower glucose values during exercise, as well as lower insulin levels following a carbohydrate meal. This effect was not seen when horses were unfit and sedentary. Other groups have shown either little, or no effect from supplementation of chromium. One study even suggested deleterious effects on the exercise capabilities of horses when supplemented with chromium. At this point in time, the exact amount of chromium horses should receive is unknown. There may be a benefit to feeding chromium to a select group of horses that have demonstrated insulin resistance, but widespread supplementation may be unwarranted, or even harmful to the general population.

On a final note, remember that many minerals interact in very complex ways in the body. Indiscriminate supplementation of one mineral may upset a very delicate balance in the ability to absorb and utilize other minerals. While data in other species may indicated positive benefits of supplementation, it is best to address supplementation in horses thoughtfully, with the goal of providing a proper diet to the animal, which only enhances, and does not detract, from their health.

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