Selenium - just how important is it?

Selenium levels in soil are low in many parts of New Zealand, especially in the Manawatu region, which means that Selenium levels are low in grass, hay and haylage. Since forage is the main natural source of selenium for horses, many horses become selenium deficient over time if they are not supplemented appropriately. 

Selenium is a trace element, meaning that it’s a mineral that only needs to be supplied in the diet in tiny amounts. Selenium plays an important role in the maintenance of cell integrity, growth, reproduction, and immune response. A selenium deficiency can cause many issues, some of which are easily recognized (such as tying up). However, the effects of a selenium deficiency can also be very insidious. Things like low immunity, symptoms of early aging and low general health evolve gradually and owners often struggle to notice these things over time. Broodmares that are grazed without supplements may have low pregnancy rates, but the poor fertility often gets blamed on other things. As vets, we try to look out for signs that a horse might be low in essential nutrients. Signs of low general health can be seen in bad hoof quality, low hair coat quality, ill thrift, poor performance, fertility problems, behavioural issues, etc. 

Many horses in New Zealand are grazed without supplementary feed because New Zealand grows such an abundance of grass. Some horses may be partially supplemented with minerals such as magnesium-oxide or salt. 

Nutrients are the building blocks of our body. The following may sound obvious, but as a rule, we only receive the nutrients that are contained in the food we eat and of course, the same is true for horses. When horses only eat the relatively deficient grass and hay that we are able to grow in New Zealand, or they are only partially supplemented, nutrient deficiencies will build up over time. This can also happen when horses are supplemented with a lick, because the amount of nutrients that they take up will differ every day, depending on how much the horse feels like consuming. Supplements that are fed as pellets or powders may also lack the right nutrient quantities, or the nutrients may be out of balance which can influence uptake. There are many reasons why supplements all have different compositions. Every company will have their own approach to producing their supplements to add to a horses’ diet. Horses are all kept and managed in different ways, and not every supplement suits every situation. The only way to check whether a supplement is actually supplying your horse with enough of the nutrients it needs, is to look at what the horse’s diet is composed of and compare the specific product’s contents to the horse’s dietary needs. These have been established by scientific research and are different for cold-blooded or hot-blooded horses, mares/geldings versus stallions, spelling horses versus horses in training, young horses that are growing and mares that are pregnant or nursing. 

Many horse owners might already know or have heard that this is a selenium-deficient area, but not everyone knows that selenium is only one of a few minerals that are important for immunity and general health. Just like selenium, these other minerals can also be low if a horse is not appropriately supplemented. For example, zinc, copper, and manganese protect the body from free radicals, just like selenium.

Luckily, we can test a horse’s selenium status by examining their blood. There are different ways of doing this. By testing how much free selenium there is in the blood (serum), we get a measure of the very “acute and present” state in the horse. However, this value quickly changes in response to uptake and use, meaning that it constantly fluctuates (just like a person’s blood sugar levels change constantly) and therefore only tells us something about the short-term selenium status. A better, more reliable test for selenium status measures the concentration of an enzyme (called GSH-Px) in the blood that makes use of selenium. If there’s not enough selenium in the body, the concentration of GSH-Px goes down as well. The GSH-Px value gives us a measure of the slightly more long-term (about three months) selenium status in a horse. The majority of selenium in the blood sits in the red blood cells as GSH-Px, which is why for GSH-Px a “whole blood” sample is tested instead of blood serum. Our lab measures both free selenium and GSH-Px to give us an indication of a horses’ selenium status. 

If other nutrients such as zinc and copper are that important too, then why don’t we measure them in the blood? Just like selenium, measurements of free minerals in the blood are not always very reliable. Their levels in the blood fluctuate constantly. For selenium, we have GSH-Px to help us out, but for most minerals, a liver biopsy would be needed to say more about their long-term status. However, if your horse normally doesn’t get supplemented, or is only partially supplemented, and blood selenium levels appear to be low, then it can be safe to assume your horse would benefit from an all-around supplement.

What happens if selenium levels are too low? Strenuous exercise is known to induce oxidative stress, leading to the production of free radicals. Free radicals can damage cell membranes and therefore cause tissue damage in muscle and lungs, two organs that sports horses very much depend on. As a group, anti-oxidants fight off damaging free radicals, reducing tissue damage. If an individual doesn’t have adequate amounts of anti-oxidants (as with a selenium deficiency) there is more damage to cell membranes, which causes sub-optimal and delayed recovery from strenuous exercise. It also increases the risk of tying up and, in general, causes the body to age faster. 

The function of selenium is linked to vitamin E. Selenium that sits in the fluid around cells will remove lipid peroxides formed by free radicals. Vitamin E sits in the cell membrane and decreases the formation of lipid peroxides by free radicals. A selenium deficiency can therefore partially be compensated if there is enough vitamin E and vice versa. However, adequate amounts of both is needed to minimise oxidation-induced tissue damage.

So, imagine you’ve had your horse’s blood tested, and the results have come up as too low. The first step would be to review your horse’s diet to find out whether you should be supplementing selenium, or a combination of other nutrients as well. With most nutrients, if a horse gets too much it will excrete what it doesn’t use. That doesn’t mean that we should give a horse an overdose of everything just because we can, because, besides the extra cost of the supplements, the liver and kidneys do have to work harder to metabolise and excrete everything the body doesn’t need.

A little bit too much is not a problem, and a healthy horse can handle these things just fine. One supplement we do have to pay attention to though is selenium. An overdose of selenium is toxic to the body, which is why a horse should not get more than 1 mg of selenium per 100 kg body weight per day. However, a horse does need about 0.3 mg of selenium per 100 kg bodyweight daily to stay healthy. This means that if we’re trying to supplement a horse that’s low on selenium, we can give it a slightly higher dose of selenium daily, but we must make sure we don’t give it too much. By checking and considering all the different sources of selenium that the horse consumes in a day and calculating the dose we can safely add to the diet, we can make sure a horse will replenish its selenium without any of the risks of intoxication.

If you are more interested in having your horse’s diet checked out completely, we can work out what your horse’s dietary needs are and look at the products you’re using to see whether your horses’ diet is attending to its needs.

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