DELIVERING COST EFFECTIVE SELENIUM SUPPLEMENTATION TO RUMINANTS


By Laura Drury, Ruminant Technical Advisor, Trouw Nutrition GB


As cattle and sheep farmers continue to strive for improved production efficiency, the likelihood of animals succumbing to nutritional and physiological stress has never been higher. The inclusion of a quality source of organic selenium in mineral supplements can have a significant effect on the performance of these animals, delivering a positive return on investment over inorganic sources.


Selenium is an essential micro-nutrient for both humans and animals, having numerous essential roles in the body. One of its primary roles is in antioxidant function via regulation of one of the body’s primary antioxidants, glutathione peroxidase, which metabolises reactive oxygen species and helps reduce oxidative stress. If there is insufficient selenium in the body then DNA, proteins and lipids can be damaged as a consequence. Selenium’s role in antioxidant activity is also complementary to that


of another major antioxidant: vitamin E. While vitamin E has a protective role at the cell wall level, selenium operates at an intracellular level, so an animal’s diet should be balanced for both of these nutrients since they are not interchangeable. Currently, the major source of selenium supplemented in animal


feeds is from inorganic sources, usually sodium selenite. One of the major disadvantages of this source, however, is that it can be reduced into non-absorbable compounds along the gastrointestinal tract which therefore reduces its absorption. This is especially the case in ruminants where inorganic selenium is highly susceptible to dissipation forming elemental selenium in the rumen which has a negative effect on bioavailability. In addition, any excess inorganic selenium supplied to the animal cannot be stored, and is therefore eliminated in urine and faeces having a negative economic and environmental impact. In comparison, organic sources of selenium are a relatively new


form of supplementation, first introduced in the 1990s. The elemental selenium is bound to specific amino acids, forming selenomethionine (SeMet) and selenocysteine (SeCys). These selenocompounds are actively absorbed across the intestine, like amino acids, leading to higher retention in tissues and bioavailability. Higher retention in tissues is one of the major advantages organic selenium has over inorganic sources, which ultimately leads to reduced urinary and faecal losses. With reference to maximum permitted levels, the legal maximum


total selenium in a ruminant diet is 0.5mg per kg of complete feed with a moisture content of 12%. Of this, a maximum of only 0.2mg per kg can be supplied in organic form. With limitations on how much selenium can be provided in the


diet, the objective of dietary formulation must be to provide the dietary selenium in the most bioavailable form to optimise the effective supply


to the animal. Given the superior bioavailability and the ability to be retained by the animal, organic selenium is considerably more efficient than inorganic sources.


Determining organic selenium quality There are numerous sources of organic selenium available on the market so it is important to make a considered judgement based on all aspects of quality. The most widely available and proven sources of organic selenium are selenium yeasts. To be sold in Europe as a nutritional feed additive, a selenium


enriched yeast has to adhere to strict requirements. They must have a total selenium content of 2000-2400mg Se/kg, with 97-99% of the selenium in organic form with a minimum 63% of this as SeMet. SeMet is one of the most efficient sources of organic selenium,


being both readily absorbed and stored. It is also the only organic form that is easily and routinely analysed, making it the best, and most widely used tracer for selenium yeast quality. Nonetheless, this does not make SeMet a superior source of selenium, with SeCys also of biological importance. SeMet is mainly incorporated into ‘non-specific’ body protein such


as muscle and milk, and may therefore only serve a storage function without any further functionality unless called upon during periods of stress, and in which case it is then metabolised to SeCys. SeCys, however, is used to build selenoproteins that have


‘specific’ functions in the immune system, antioxidant defence and thyroid function, such as the enzymes glutathione peroxidase and thioredoxin reductase. Inorganic selenium also follows a similar route to SeCys after


absorption. However, differences occur in stability prior to absorption. Whilst inorganic selenium can be reduced to non-absorbable compounds in the rumen, SeCys remains stable. Furthermore, within a selenium yeast, the selenoamino acids are bound into the protein of the yeast cells so they are not isolated. By being incorporated in the protein matrix they are afforded protection from degradation. SeMet content can also give a good indication of the quality


of processing. When it is higher than 63%, this is evidence that the manufacturing process has been well controlled which is necessary to maximise the uptake of selenium by the yeast cells. If the SeMet content is low, this is a likely indicator of poor control of the production process, therefore resulting in a poorer quality end product. The physical appearance of selenium enriched yeasts can also


give an indication of production control and product quality. Yeast is comprised of around 50% amino acids, and anything that damages the proteins during manufacture will reduce digestibility and therefore compromise product quality.


FEED COMPOUNDER MAY/JUNE 2019 PAGE 45


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