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.
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.
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!