Nutrition: Selenium – More Than Meets the Eye
By J.A. Davidson P.Ag. PAS Bar-D Agri-VenturesFeatures Nutrition and Feed Research
The Selenium source is important
The importance of Selenium in the nutrition of livestock and poultry has been recognized since 1957 when it
was first recognized as an essential micronutrient.
The importance of Selenium in the nutrition of livestock and poultry has been recognized since 1957 when it was first recognized as an essential micronutrient. It was in the early 1970s that supplementation of animal diets with either sodium selenite or selenate became an accepted practice worldwide.
Selenium is a metalloid with chemical properties similar to sulphur. It was first discovered in 1818 in Sweden by J.J. Berzelius. There are 77 stable elements that occur naturally. Selenium is one of the least abundant of the 77 with an overall ranking of 65th. It is estimated that worldwide Selenium has an average concentration of only 0.05 mg/kg in the soil. The amount of Selenium present in soil for absorption by plants is directly related to the amount of selenium in the rock that was weathered to form the soil.
The concentration of selenium in any given soil and the soil’s physical properties have a direct impact on the amount of Selenium in plants. Soils that are alkaline and well aerated tend to produce plants with higher Selenium content.
These types of soils can actually produce plants containing toxic levels of Selenium. Acidic soils are reported to result in lower plant Selenium and acidic soil types infrequently produce plants with high Selenium concentrations. The common definition of soil Selenium status is Very high > 1.5 mg/kg air dry soil, High > 0.9, Low < 0.5 and Very Low <0.3. Unfortunately the uptake of Selenium by plants is highly dependent on environmental and plant specific factors. An adequate soil Selenium reserve does not inevitably translate into adequate plant reserves.
The feed industry has supplemented with Selenium for just over 30 years. As a result many producers have never experienced the effects of a Selenium deficiency. It is important to remind the livestock and poultry industry of Selenium’s many functions.
Primarily Selenium is recognized for its role as a component in the enzyme glutathione peroxidise. This enzyme
protects the body against oxidative damage to cellular and sub cellular membranes by converting hydrogen peroxide to water and liquid hydroperoxides to alcohols. Selenium also acts as an antioxidant that destroys harmful peroxide molecules and lowers the cellular depletion rate of Vitamin E required to shield lipid membranes. Adequate Selenium supplementation has been shown to aid in the retention of Vitamin E in blood plasma.
Selenium has also been proven to provide protection from some toxic elements such as arsenic, cadmium and mercury. Other reported functions include stimulating antibody-producing cells, improving growth rates by influencing the production of the hormone Thyroxine, converting methionine to cysteine and aiding in the repair of damaged DNA.
Generally Selenium acts as a natural antioxidant, promotes growth and reproduction, enhances the immune system and viral infection resistance and is closely linked with Vitamin E function.
Since supplementation of livestock and poultry feeds started, the most common manifestations of Selenium deficiency have become less common. The classic signs of a Selenium deficiency are nutritional muscular dystrophy, myocardial diseases and poor disease resistance with impaired immunity. In poultry the most evident signs of a Selenium deficiency are reduced egg production, reduced fertility and hatchability with severe deficiencies leading to pancreatic failure, nutritional muscular dystrophy and encephalomalacia.
Is There a Problem?
The significance of Selenium in livestock and poultry feeds was recognized in the early 1970s and after supplementation with sodium selenite became accepted everyone stood back, took a deep breath and considered the problem solved. The incidence of the primary indicators of selenium deficiency dropped significantly. But in many ways things had not really changed. The potential for selenium-related problems to occur still exists. Huge areas of North America produce
forages and grains that are considered to be Selenium deficient. Of particular interest relative to the Selenium content of forages and cereal grains are large areas of Alberta and virtually all of British Columbia that are rated as being deficient. Everything in Canada east of the Manitoba – Ontario border is considered to be marginal to deficient. The most south eastern section of Alberta and the southern areas of Saskatchewan and Manitoba are considered generally adequate however that definition infers that approximately 80per cent of grains or forages will have sufficient Selenium. This leaves 20 per cent open to question.
The simple act of living where soils are considered to provide adequate Selenium will not ensure that the diet of domestic animals contains adequate Selenium. In the fast paced world grains and forages move hundreds to possibly thousands of kilometres from producer to end user. It is not uncommon for grains or forages grown in Alberta to be consumed in Ontario. In many years forage crops are moved from areas with abundant supplies to places suffering the effects of either drought or excessive moisture. The feed your animals consume may have a significant proportion of the ingredients sourced from other regions of Canada or the U.S. Supplementation on a continual basis, irrespective of local conditions, will ensure a basic level of protection from the most common Selenium-related conditions.
While the naturally occurring Sele-nium status of feedstuffs has remained relatively constant (possibly declining on a long-term basis) the characteristics of the livestock being fed have changed dramatically since the 1970s. The genetic growth potential of many farm animals has increased at an incredible rate. Feed conversion and rate of gain have improved significantly in poultry. Egg production has increased both in hen day production and total egg mass produced. The improved growth rate of swine has been impressive while geneticists worked to reduce back fat. Dairy cattle produce more milk on less feed. Over the last 35 years the productive life expectancy of food-producing animals has been extended through genetics and nutrition. The current high metabolic rates of our food producing livestock and poultry require a reassessment of their micronutrient requirements.
Unfortunately, with these types of improvements, we have to recognize some negative impacts. Animals performing at optimum genetic potential are much more sensitive to both physiological and environmental stressors. Stress contributes appreciably to breakdowns in the immune system, poor reproductive efficiency and myocardial diseases. All of these problems have a relationship to selenium supplementation and adequate intake of nutritionally available Selenium.
The key words in the last sentence are “adequate intake of nutritionally available selenium.” Supplementation of livestock or poultry diets is generally limited to 0.30 mg/kg of complete feed by most regulatory agencies through out the world. Since Selenium was cleared the primary source of supplemental selenium had been sodium selenite. While contributing immensely to the improvement in livestock and poultry performance this level and source of supplemental Selenium does not guarantee optimal Selenium tissue levels and optimal performance levels are ever achieved. Sodium selenite has the potential to be highly toxic and this limits our ability to supplement fortified rations.
Poultry manifest selenium toxicity through reduced egg production, lower hatchability and the presence of
embryonic deformities. The supplementation of selenium with sodium selenite is a balance between adequate and excessive. This narrow window of supplementation inhibits our ability to formulate feeds that are not simply adequate but contribute to optimum performance.
Over the last decade, and more notably the last five years scientists have found that achieving a Selenium intake that optimizes tissue and circulating blood levels can have an enormous effect on livestock performance. Most recently through the science of Nutrigenomics, which focuses on the effects of individual nutrients on gene activation or suppression, scientists have found that there is an untapped potential for improving animal production through feeding individual genes optimal levels of biologically available supplemental selenium.
Earlier we keyed in on the phrase “adequate intake of nutritionally available Selenium.” In order to achieve the full potential of Selenium supplementation we have to move beyond simple adequacy to a point where we have optimum Selenium levels in a form that will permit us to actually have an effect on individual genes. This cannot be achieved with limited use of a potentially toxic compound like sodium selenite.
Organic Vs. Inorganic
In the 1970s when Selenium was cleared for use in feeds, the two acceptable forms were sodium selenite and sodium selenate. These are inorganic sources of Selenium. These sources were chosen because they were readily available and also inexpensive. Sodium selenite is a byproduct of the refining of copper, nickel and cobalt and is easily available worldwide. Neither product is well absorbed by the body and the majority is excreted before it is absorbed. Fortunately there is sufficient absorption to remedy the most basic deficiency problems, however inorganic Selenium is not 100 per cent effective in resolving all Selenium-deficiency conditions.
One major reason for inorganic Selenium’s inability to be totally effective is that inorganic Selenium provides only immediate relief from a deficiency. Inorganic selenium is not stored in the body and any inorganic Selenium supplied in excess of immediate needs is excreted via the urine. We know that in mammals inorganic selenium does not transfer efficiently across the placental wall or through colostrum and milk. As a result, while the dam may appear to be adequately supplemented with Selenium the offspring may have suboptimal reserves.
As mentioned earlier one of the greatest concerns with inorganic Selenium is potential toxicity. Toxicologists measure the toxicity of chemical substance with a measure called LD50. LD50 measures how much of a given substance it takes to kill 50 per cent of test groups of subjects. The number is generally reported in a dose rate of milligrams per kilogram of body weight. The lower the LD50 number, the more toxic the chemical is considered to be. For reference purposes the LD50 of sodium selenite is considered to be between 5 and 50 mg/kg of body weight.
In recent years a new source of Selenium has become available to the feed industry. This form is considered to be an organic chemical form. Organic Selenium is derived from the culture of specific strains of yeast (Saccharomyces cervisiae) in a Selenium-rich environment. The yeast cells absorb selenium salts from the growth media and incorporate them into selenoamino acids in place of sulphur. The most common of these selenoamino acids is selenomethionine with research showing approximately 60 per cent to 90 per cent of amino acid bound Selenium in the selenomethionine form. Plant feedstuffs (including yeast) have been reported to contain over 20 different selenoamino acids. Selenomethionine has been recognized as the most important form responsible for transferring Selenium from Selenium accumulating plants through to herbivores and subsequently carnivores or herbivores. Selenomethionine is the organic form of selenium that passes through the placenta and also to offspring through milk and eggs. Unlike inorganic forms of Selenium, selenomethionine can be and is stored in animal tissues, thereby providing long-term supplies of highly bio-available Selenium.
Selenomethionine is Mother Nature’s storage form of Selenium. Plant Selenium is stored as selenomethionine and once ingested animals can store Selenium as selenomethionine and build body reserves. Forage plants and grains store Selenium in varying amounts based on soil availability. Selenium yeasts can be selected specifically for their ability to incorporate Selenium into a selenoamino acid. By selecting specific high accumulating yeast varieties and culturing them in a controlled Selenium environment it is possible to produce Selenium yeasts with very consistent levels of selenomethionine. These yeasts can be incorporated at fixed rates into livestock or poultry feeds to ensure optimal Selenium fortification. Selection of yeast strain is critical to the effectiveness of Selenium yeast. Studies of different yeasts have shown a range of 11.5 per cent to 28 per cent in water soluble Selenium and 15.5 per cent to 72 per cent water-insoluble polysaccharide-bound Selenium. This data illustrates that not all Selenium yeast products are equal and the effectiveness of different Selenium yeasts may not be the same. Selection of the right Selenium yeast source is critical.
One yeast product that has been recognized worldwide as being highly bio available and of consistent strength is Sel-Plex™ from Alltech (Alltech Biotechnology Inc., Lexington, Ky.) Of primary importance is research illustrating that Sel-Plex™ with Selenium in Mother Nature’s preferred form of selenomethionine is considerably less toxic than inorganic forms of Selenium available to the feed industry. Tests conducted on rats and mice demonstrated that the LD50 for Sel-Plex™ is greater than 2,000 mg/kg body weight. Based on this data Sel-Plex™ is potentially between 50 and 500 times less toxic than sodium selenite with a LD50 between 5 and 50 mg/kg of body weight.
Selenium provided in the form of selenomethionine from Sel-Plex™ has been shown to provide numerous advantages over inorganic Selenium. These advantages are the result of optimum supplementation and bio-availability. n
Organic Selenium – The Proof is in The Performance
Research in a laboratory environment gives nutritionists a set of expectations for what will happen in the real world of farming. Only when we see results in farm scale tests can we truly know things work. In poultry opportunities for improved production are available to all facets of the industry.
Improved Selenium status through the use of organic Selenium like Sel-Plex™ results in improved feathering, enhanced growth rates and feed conversion ratios and lowered incidence of ascites-induced mortality. Canadian research showed a drop in ascites related mortality from 10 per cent in unsupplemented birds to 2.8 per cent in Sel-Plex™ fed birds. Breeder operations report more eggs per hen housed, over 2.0 per cent improvement in hatchability coupled with a 1.6 per cent increase in fertility and the overall dead in shell numbers reduced by 1.45 per cent.
The supplementation of layer hen diets with Sel-Plex™ has been shown to extend the shelf life of table eggs. As eggs age the white thins and the yolk tends to flatten out. Using Haugh unit measurements it has been shown that adding selenomethionine to the diet significantly increases the Haugh unit value over selenite supplemented eggs. Japanese research showed eggs from selenite fed hens dropped to below 75 within four days of storage while eggs from Sel-Plex™ fed hens graded above 75 past 7 days of storage. Whites stay thicker and yolks remain firmer under Sel-Plex™ supplementation.
Similarly Sel-Plex™ can improve the quality of processed poultry meat. Drip loss is a phenomenon where poultry meat looses weight due to its inability to retain water. As much as 3 per cent of the weight of processed poultry meat can be lost between the processing plant and the consumer. Australian research demonstrated that supplementation with organic Selenium like Sel-Plex™ reduced the 24 hour drip-loss in broilers from 21 to 26 per cent. It is believed that this occurs because organic Selenium reduces cell membrane damage that allows water movement out of the meat.
Nutritionists currently recommend replacing between 50 per cent and 100 per cent of supplemental Selenium with organic selenium in the form of Sel-Plex™ to maximize performance and profits.
In the end, there is a definite need for selenium in modern poultry production. Our birds are constantly being pushed to produce more eggs and put on more weight so it is important our feeding program include organic selenium to help optimize production.
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