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Selenium (Se) is a micronutrient that is not essential for forage growth but is essential to the health of animals that graze the forages. Parent rock material in western Oregon is deficient in Selenium; therefore, our soils lack this micronutrient. Selenium is needed to maintain good health of cattle. Cows fed inadequate amounts of selenium are at higher risk for white muscle disease, retained fetal membranes, reduced immune system response, and mastitis. Current US FDA regulation allows ruminant diets to be supplemented with 0.3 ppm selenium from either sodium selenite or selenate.
Selenium has commonly been supplemented in livestock through injections and mineral mixes. These strategies often do not provide adequate blood Se levels for an extended amount of time and are expensive. Injections provide only short-term increases in Se blood levels (about 30 days), and animal consumption of mineral mixed is inconsistent. Mineral mixes commonly contain inorganic forms of selenium, poorly utilized by the animal. However, plants actively take up selenium and convert it to organic forms that are easily and effectively utilized by animals. Therefore, the addition of Se to commercial fertilizers can help reduce deficiency of Se in animals.
Recent studies by Amy Peters (OSU Extension) in Coos Bay County found that the addition of Se as fertilizer in amounts of 0.5 to 2 pound per acre selenium increased selenium content in the forage clippings. Pasture clippings showed adequate levels of Se for two straight years. This study also found that animals grazing in Se fertilized fields for even a short amount of time had adequate blood Se levels in their systems. Work at the Union Experiment Station showed that even lambs and calves from dams grazing the pasture had adequate blood selenium levels. Selenium is consumed with each bite of pasture or hay. Selenium applied with fertilizer is an effective way to provide adequate levels of dietary Se for livestock.
Cost and Availability
Fall is the best time to apply fertilizers and time to perform that long awaited soil test to see if you need to supplement with selenium in your fertilizer mix. Selenium, as an added ingredient to fertilizer, is available commercially at a cost of about $2.50 - $4.0 per acre. Such fertilizer will sustain adequate levels of Se in forage for approximately two years. This is more cost effective than using mineral blocks and injections. Some fertilizer dealers in western Oregon market a commercial preparation called Selcote which is a mixture of readily available Se along with encapsulated, slow-release Se. In this way, some Se is immediately available to the plant, while the remaining Se is available over an extended period of time. Private producers are using this approach and are finding that Se levels in their livestock blood have improved already. A word of caution though is that you should not provide other forms of supplemental Se if you fertilize your pastures with Se every other year. You should, however, continue to provide other minerals needed by the animals. It is a good idea to include blood mineral level screenings during routine health exams on your livestock, not only for Se but also for other required minerals.