Selenium in Horses

by Bas Van Luijk,

Bas is the Head Veterinarian at East Coast Farm Vets

Selenium levels in the soil are low in many parts of New Zealand but historically the East Coast area hasn't dipped below the threshold. However, over the last few years we have seen the occasional case of severe selenium deficiency across sheep, cattle and more recently, horses.

Selenium levels can drop in grass, hay and haylage if not rectified by fertiliser. Since forage is the main natural source of selenium for horses, many horses become selenium deficient over time if they are not supplemented appropriately.

What is selenium?

Selenium is a trace element, meaning it’s a mineral that 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 lower pregnancy rates, and are more at risk of retaining their membranes after giving birth.

General health problems can be seen in bad hoof quality, low hair coat quality, ill thrift, poor performance, fertility problems and behavioural issues.

Selenium in the diet

Many horses in New Zealand are grazed without supplementary feed because New Zealand grows such an abundance of grass.

Nutrients are the building blocks of the body and are an essential part of the horse's (and other animal's) diet. If their food (often mainly grass) is deficient 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.

Scientific research has established the trace element requirements for horses, and they are different for cold blooded/ hot blooded horses, mares/geldings versus stallions and pregnant mares.

Can we test selenium levels?

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.

To evaluate the selenium status over a longer period of time, it's best to investigate 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 (approx. 3 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.

What happens when 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 sport 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.

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 visa-versa. However, adequate amounts of both are needed to minimize oxidation-induced tissue damage.

If selenium is low, what can I do?

Many fortified feeds have added minerals and can have adequate amounts of selenium, provided you are feeding as per the recommended feeding rates. This is where a lot of people can go wrong, especially if they're trying to balance their horse's weight and not feeding as per the instructions. It pays to have a look at the ingredients in the feed and the nutritional analysis to determine whether the feed is specific for what you're looking for.

If you're balancing weight gain with easy keepers, or have a multitude of horses to cater for, a mineral balancer pellet could be a better option for you. Low calorie mineral balancers for other minerals, especially iodine, copper and zine as well as selenium are ideal for horses with plentiful grass and no need to hard feed.

If you're unsure, best thing would be to talk to one of us, we can have a look at your horse's diet and go from there. An overdose of selenium is toxic to the body, so it pays to get it right. The safety margin for selenium is very narrow and not something you want to experiment with.

If you are 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 horse's diet is covering all bases.

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