| Selenium Supplementation for Sheep By Gene Pirelli Many parts of the world, including Oregon, have soils that are deficient in the essential micronutrient selenium (Se). A severe selenium deficiency results in "white muscle disease.” Subclinical selenium deficiency causes problems like muscular weakness of the newborn, reduced weight gain and reproductive problems. There are two forms of selenium approved for use in livestock feed: selenite and selenate. The form used for injection, feed, mineral or boluses is usually inorganic sodium selenite. One limitation of using inorganic selenium is the apparent short duration of storage in the animal. Selenium injections usually provide adequate levels for 30 days or less. Additionally, some flocks may not have access to selenium salt-mineral mixes for extended periods of time, and therefore the sheep may be selenium deficient by the end of the grazing season. Over 10 years of research at Oregon State University cleared the way for the approval of selenium enriched fertilizer in Oregon. The pasture plants take up selenium and convert it to an organic form, which is more available to the animal. The intake of the organic form also appears to have the advantage of longer storage in the animal. The recommended rate of application is one pound of Selcote (a pelleted selenate product containing 10 grams of selenium per kilogram) per acre when mixed with commercial fertilizer. One question often asked by producers is how to avoid selenium deficiency caused by limited or no access to mineral during the grazing season. Can selenium be safely built up in the animal and used later? To come up with answers, the Oregon Sheep Commission approved funding for a project in 2007-08 to test the application of supranutritional levels of selenium to pasture through fertilization. The goal was to determine if a short grazing period on this forage would result in sufficient long-term selenium storage in ewes. The project began with the application of three pounds of Selcote per acre to 15 acres of pasture at the OSU Sheep Center on February 1, 2007. The Selcote was mixed with nitrogen fertilizer. Fifty-eight days after fertilization, 30 ewes were sorted randomly into two groups. One group of 15 ewes was moved to the selenium fertilized pasture. This group had no mineral supplementation. The second group was also moved to pasture (non-fertilized) with a 200 ppm selenium sheep mineral mix. The selenium concentration (on a dry matter basis) when the sheep began grazing the test pasture was 1.46 parts per million (ppm). As a reference point, the average selenium content of the non-selenium fertilized pastures at the sheep center range from 0.07 ppm to non-detectable. The ewes grazed for 40 days on the pastures. The two groups of ewes were then combined and treated as a single grazing group with no mineral supplementation. Blood samples were taken from the ewes every four weeks for nine months for selenium analysis. . Whole-blood selenium concentrations were higher immediately post treatment in sheep grazing selenium-fertilized forage (0.573 ppm) compared with sheep consuming the mineral supplement containing selenium (0.286 ppm), and remained higher for approximately nine months. Normal selenium blood levels can range from a low of 0.07 ppm to 0.250 ppm or more. The 0.07 end of the range may be sufficient to prevent white muscle disease, but is too low for adequate immunity and reproduction. We concluded that short-term exposure of sheep to supranutritional selenium-fertilized forage results in increased whole blood selenium sufficient to maintain adequate concentrations throughout grazing periods when there is limited access to selenium supplements. Targeted grazing of selenium-fertilized forage provides another tool for sheep producers to maintain and improve the health and productivity of their flocks. This is significant when seasonal grazing practices result in limited access to selenium-containing salt-mineral mixes for extended periods of time. We do not recommend exceeding the application rates of selenium mentioned in this article. Organic forms of selenium are considered safer, but toxicity may still occur at high levels. Testing of forage and blood is also suggested to avoid problems. Research Project Team Members: Gene Pirelli, OSU Extension, Department of Animal Sciences Tom Nichols, manager, OSU Sheep Center Jean Hall, DVM, College of Veterinary Medicine Read the More About Selenium Check out Oregon Sheep Commission Page
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