Written By Dr. Kris Hiney

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

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

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

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

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

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

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

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

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

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

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

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