Delivering Cost Effective Selenium Supplementation To Ruminants

30 May 2019

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.

For example, a lighter colour product is an indicator of higher digestibility.  Whilst rapid drying can help reduce production costs, it can also ‘burn’ the protein in the presence of residual sugars therefore reducing digestibility, which can be observed with a darker colour.

A more granular product also indicates the presence of more live yeast cells which have been shown to have minimal absorption.  Selenium yeast should be inactivated with autolysis of the yeast proteins, helping to make them more accessible leading to improved selenium absorption.  A high quality selenium yeast product should therefore have the appearance of a fine spray-dried powder. 

Additionally, to help identify a good selenium yeast, a Certificate of Analysis should confirm that the total selenium and SeMet contents conform to the guaranteed specification.

Trouw Nutrition’s Optimin SeY is a premium high quality selenium-enriched yeast containing 100% organic selenium comprised of an optimal combination of natural organic selenium forms including SeMet as the main bioavailable form plus SeCys and other organic selenocompounds. The product also offers guaranteed and consistent quality, with Trouw Nutrition being the only supplier of selenium-enriched yeast that runs analysis on both total selenium and SeMet on a routine basis for each batch released and guarantees these values on a certificate of analysis.

Furthermore, analysis of multiple batches of Optimin SeY has shown an average selenium content of 2201ppm with 66% of this selenium as SeMet, both exceeding the minimum EU specification requirements for a selenium yeast.


The stability of different organic selenium sources can also vary depending on their origin and matrix, with differences in stability having been shown between selenium yeast sources and synthetic organic selenium sources.  Furthermore, the presence of trace minerals in a premix can affect the stability of other premix components.  Both the stability of selenium as well as SeMet present in Optimin SeY have been shown for three years for Optimin SeY itself in pure form, one year in premix and six months in complete feed, which is critical to ensure maximum benefits are achieved both in terms of cost effectiveness and animal performance.

Selenium-enriched yeasts in ruminant diets – a cost effective supplement

Due to its function within the immune system and antioxidant status, the selenium content of a diet is particularly important during times of stress such as the transition period in dairy herds, pre-calving in suckler cows and pre-lambing in sheep.

The possible drawback of selenium yeast however is that it is more expensive than inorganic selenium sources, leading to an increase in mineral supplementation costs and finished feed prices.  With many ruminant farming systems facing a financial squeeze, it is important to be able to demonstrate a return on investment from the addition of organic selenium.

The marginal cost of adding organic selenium to the legal limit is actually very small.  For example over a six week transition period, the cost of supplementing Optimin SeY would be less than £1 per head more than supplementing with just inorganic selenium.

When this is compared to the known problems and cost implications of diseases associated with selenium deficiency, the cost soon becomes less of an issue.  For example, industry figures estimate that each retained cleansing case costs on average £290, while each case of metritis costs £189.

Research also shows that 60% of clinical mastitis cases during lactation can be traced back to bacterial infections that occurred during the dry period (figure 1), and the risk of picking up new infections is 10 times greater during the dry period than during the lactation.

 Figure 1: The origin of infection (dry period or lactation) in clinical mastitis cases

Research has shown a positive effect of supplementing Optimin SeY versus inorganic selenium on blood selenium concentration and glutathione peroxidase (GPx) activity in dairy cows.  Cows supplemented with Optimin SeY were shown to have a blood selenium concentration 84% higher, and GPx activity 1.3 times greater than in cows supplemented with sodium selenite.  In the same study, supplementation with Optimin SeY over an 8 week period reduced the number of udder quarters with positive bacteriology and significantly reduced the somatic cell output per milking session.

Multiple trials over the last 20 years have also confirmed the positive effects that Optimin SeY can have on milk quality, having repeatedly been shown to reduce somatic cell counts, positively impacting on milk sold and milk price (figure 2). However it is important to remember that the cause of a high somatic cell count can be multifactorial, although a higher selenium status can be beneficial.


Figure 2: The effect of Optimin SeY supplementation on the percentage reduction in somatic cell counts

Feeding transition cows with a more effective source of selenium also has benefits for calves with an improved supply of selenium both in utero via the placenta and after birth through colostrum and milk. This not only helps support the process of brown fat mobilisation and prevention of hypothermia, but also the immune and antioxidant status of the calf leading to better health, performance and survivability.

In beef cattle, Optimin SeY has been shown to not only improve antioxidant status but also increase selenium concentrations in the plasma compared to supplementation with inorganic sources. When the same animals were slaughtered, drip loss from carcases was reduced by 25% while meat lightness and tenderness were also increased.

 As farmers look for ways to improve efficiency to reduce costs of production, supplementing diets with the optimum content of high quality organic selenium can have a positive impact on key parameters affecting health, production and fertility delivering a significant return on investment.

 For further information on Optimin SeY, please contact the Trouw Nutrition GB technical team: T: 01335 341102 E: