For centuries, selenium was considered a toxic element. In recent decades, however, its advantages have also been discovered. Selenium is a useful element in animal nutrition, such as for poultry, but proper use is essential.
By Dr. Frank Edens , Department of Poultry Science, NC State University, Raleigh, NC, USA, and Dr. Ted Sefton , Alltech, Guelph, ON, Canada
As with so many things in the world, the better the design of a specific item to meet a specific need, the better the response will be. We see this typically in race cars, sailing boats and nearly all aspects of performance. It is no surprise, then, that the design for the form of selenium illustrates the same issue when considered in animal production. Understanding the development of our knowledge on selenium is very important if we are to appreciate our position today, and what the future holds for this essential trace element.In about 1295, Marco Polo described a disease syndrome encountered during his travels to China when the hooves dropped off his horses after the animals ate seleniferous plants. Reports continued on the toxic effects of selenium and hair loss in humans in Columbia in 1560. Swedish scientist J.J.
Berzelius named selenium in 1818 after identifying it in sulfuric acid preparations from mining residue.In America, it has been speculated that General George A. Custer’s defeat by the Sioux Indians in 1876 at the Battle of the Little Big Horn may be, in part, attributed to selenium toxicity. This region of Montana contains forages with toxic levels of selenium. The Calvary was delayed because the horses, as the commanding officer reported, had developed a peculiar illness. Could it be true that selenium was responsible for the famous or infamous (depending on your view) defeat of the US Army.
It was not until 1957 that the rest of the story began to emerge; selenium could be a useful element. Selenium was found to prevent exudative diathesis in chicks, and liver necrosis in swine. This was quickly followed by the finding that selenium could be a preventative measure of muscular dystrophy in calves and lambs. Research into the interaction of selenium and Vitamin E became a rewarding area of research. This led to selenium supplementation in animal feeds being allowed in many countries, including the US in the early 1970s. However, not all countries allowed supplementation, and in countries where it was allowed, the level was often limited. In the US, for example, supplementation was limited to sodium selenate and/or sodium selenite, only up to 0.3 ppm.Because of the toxic effect that selenium could have on premix plant workers, Japan and Finland did not allow supplementation. Livestock and poultry producers in those countries had to deal with feed with selenium deficiencies until 1993 when Alltech developed Sel-Plex, a yeast-derived selenium supplement based on Saccharomyces cerevisiae CNCMI-3060. This could be ‘the rest of the story’. In 2000, regulatory approval was granted in the US. Approval within the EU followed in 2006.
Safety has always been a concern. Marco Polo reported toxicity before Berzelius had named selenium. Sodium selenite and sodium selenate LD50 dose is 5-50 mg/kg body weight. For Sel-Plex, Power (2005) reports the LD50 dose is greater than 2,000 mg/kg of body weight. Under the Global Harmonised System for classification and labelling of chemicals based on oral toxicity, sodium selenite and sodium selenate require labelling indicating ‘Danger – Fatal if Swallowed’; Sel-Plex requires no such labelling. Thus, the form of selenium determines function.
Egg selenium levels
Work reported by Leeson and co-workers in Poultry Science in 2008 showed that eggs from breeders fed Se yeast or another source (two sources reported as being sources of organic selenium) had greater egg selenium levels than those fed sodium selenite. Furthermore, the albumen from breeders fed Se yeast had the greatest selenium concentration, while those fed the second Se product had more selenium in the yolk.
The forms of selenium had different modes of action. The greater selenium level in the egg albumen from breeders fed Se yeast would reflect that the yeast incorporates selenium in selenoprotein, which would be absorbed by the breeder as an amino acid and so incorporated as protein in the egg. This also explains why the selenium content of the breast muscle and liver of hens fed Sel-Plex was greater than the other two selenium sources. The production methodof the other Se is not given, but obviously it is not in a selenoprotein form as the deposition differs from the selenium in Sel-Plex; accumulating in yolk, fat, not in albumen, protein. Similar work with Sel-Plex and sodium selenite also show that sodium selenite transferred proportionately more into the yolk. Again, the form of the selenium will determine the animal response.
Further research by Renema and co-workers (2003) has shown that hens fed Sel-Plex and then inseminated with sperm from males fed sodium selenite supplemented rations have greater fertility than hens fed sodium selenite. This can be attributed to the antioxidative influence on the sperm in the sperm storage glands of the hen. Further, in their trials, the Alberta workers found that there was no negative effect on hatch of fertile eggs, nor was there such a trend in other studies. Edens and co-workers (2002) have demonstrated greater numbers of normal sperm from males supplemented with Sel-Plex. The design or form of selenium makes a great difference to the bioavailability of selenium in feedstuffs. Cantor (1997) shows that plant feedstuffs have higher bioavailability than animal feedstuffs. A total of 50% of the selenium in cereal grains is in the form of selenomethionine (Olsen and Palmer, 1976). Se-methyl-selenomethione, selenocysteine and Se-methyl-selenocysteine are the other seleno-compounds in plants (Brody, 1994). Yeast is part of the plant kingdom so this greater bioavailability will be attributed to Sel-Plex and the assay of forms of selenium is similar to that of other plants. Thus, selenium for Sel-Plex will be digested similarly to other plant selenium, as selenoproteins, not as fat soluble compounds.