Selenium (Se), like the other trace minerals, is necessary to sustain life and is essential for basic physiological functions in both animals and humans. While the daily requirement for these minerals is obviously small, their importance to and impact on the health and well-being of livestock and humans are well documented in research. Fortunately, the difference between deficiency and toxicity with most of these trace minerals is believed to be fairly broad, allowing for the wide range of supplemental regimes used around the world.
Unfortunately, selenium was recognized as a potentially toxic mineral long before it was identified as an essential nutrient, thus leading to unfortunate misunderstanding of this important trace mineral. This is one of the main reasons why federal regulations are in place to ensure the animal feed industry is cognizant of the government’s concerns.
Before we go any further, let us look at the definition of Se in a medical dictionary:
“Selenium – a nonmetallic element that resembles sulfur and tellurium chemically, causes poisoning in range animals when ingested by eating some plants growing in soils in which it occurs in quantity, and occurs in allotropic forms of which a gray stable form varies in electrical conductivity with the intensity of its illumination and is used in electronic devices – symbol Se.”
Does anyone else find it disturbing that this definition only mentions toxicity while failing to mention deficiency? To further reaffirm the medical community’s misunderstanding of this essential nutrient, one only needs to look up “white muscle disease” in this same dictionary and find that it is due only to an inadequate intake of vitamin E (versus also Se).
These blatant misconceptions persist despite the fact that research has established the areas of the world affected by Se deficiency are far larger and the consequences are more economically important, than those afflicted with Se toxicity concerns.
Selenium concentrations in soil and feed ingredients
The concentration of Se in the soil and its availability to crops vary greatly from one geographic region to another. Therefore, it is not surprising that Se in crops grown on those soils is also quite variable. Early mapping showed a wide range of Se concentrations in forages and crops and classified the various regions as high, moderate or low in selenium.
A map was put together by the National Research Council in 1983 to show the Se concentrations of the various areas in North America based on a 0.1-ppm scale (see Figure 1*). The map shows that many of the Midwestern states, which are major corn and soybean producing areas, have low to variable Se content in their forages and grains, even at the 0.1 ppm level.
The soils in a major portion of the dairy regions in the United States, east of the Mississippi river and west of the Rocky mountains, are low in selenium resulting in lower Se levels in the grains and forages produced there. Combine the above- mentioned situations with the fact that even in high-soil-Se areas, soil pH, moisture level and aeration also contribute to plant Se levels, and it is easy to see why Se deficiencies have been documented in 44 states.
Selenium and its impact on production
Even though improvements in growth or milk production are rare, supplementing selenium to Se-deficient diets generally elicits a positive response in animal health. Additionally, research with dairy and beef cattle has found that selenium supplementation to Se-deficient diets or Se-deficient areas can result in:
1. a reduction in milk somatic cell counts (SCC)
2. a lower incidence andseverity of clinical mastitis
3. fewer retained placental membranes
4. improved reproductive parameters
Each of these areas can have an important impact on the profitability of a dairy farm, and by simply improving the selenium status of the animals, the economic value can be appreciated.
Selenium and mastitis
Selenium status of lactating dairy cows is related to the incidence of mastitis and high somatic cell counts (SCC). A survey of Ohio dairy farms found a strong relationship between herd selenium and SCC. The herds with higher blood selenium concentrations had lower bulk tank SCC than herds with lower blood selenium levels. The range of somatic cell counts in these farms was between 400,000 and 100,000, making them well within normal farm averages within the Ohio dairy region.
This reduction in somatic cell count not only improves milk quality, but it also can lead to additional milk production over the total lactation of a cow. Researchers have calculated that a 50 percent reduction in bulk tank somatic cell count could result in up to 400 pounds of additional milk per cow per year in second-lactation animals.
Whole blood selenium is considered the best indicator of selenium status due to the incorporation of Se into developing red blood cells. A selenium level ranging between 130 to 150 nanograms per milliliter has historically been considered to be the adequate range. Some research suggests higher blood selenium levels are beneficial for addressing specific mastitis pathogens. It was determined cows with blood selenium of 200 ng/ml or greater had 18 percent fewer mastitis infections than cows whose blood selenium levels were 150 nanograms per milliliter or less.
Selenium and reproduction
Successful reproductive performance is important to the profitability of every dairy operation, and selenium status has been shown to be important in many reproductive functions. Selenium supplementation to Se-deficient dairy cows can improve reproductive performance. Improvements observed in this trial included fewer services per conception, improved pregnancy rates at first service and fewer days to conception.
Meeting the selenium requirement of dairy cattle
The most recent edition of the National Research Council Nutritional Requirements of Dairy Cattle defines the selenium requirement for all classes of dairy cattle as 0.3 ppm. Moreover, the Food and Drug Administration has set the legal limit for supplemental selenium to dairy cattle at no greater than 0.3 ppm.
However, continuing problems with dairy cows, like the mastitis and reproductive disorders mentioned above, suggest that current practices of selenium supplementation using sodium selenite may not be adequate. NRC recommendations are generally based on providing adequate trace mineral levels to prevent measurable deficiencies, not on adequate levels to optimize animal health.
It is well documented that the currently available inorganic selenium sources, sodium selenite and sodium selenate, are poorly absorbed and utilized by ruminants. Inorganic selenium (selenite) is absorbed much less efficiently by ruminants than monogastrics. Absorption of selenite by ruminants has been reported at 29 percent and between 17 and 50 percent.
Poor absorption of inorganic selenium is likely due to the ruminal environment where oxidized selenite or selenate is in large part reduced by ruminal microbes to insoluble and unavailable elemental selenium which is excreted via the feces. Other dietary factors also influence the availability of inorganic selenium. Dietary concentrates alter the ruminal reduction capacity with high concentrate and low pH, presumably increasing the amount of inorganic selenium the microbes would make unavailable to the animal.
This variation in rumen acidity may be one reason why the response to a given amount of selenite can vary from farm to farm in the same region. Other minerals, including sulfur and iron, can also interfere with selenium absorption.
Improving the selenium status of dairy cattle
Given the legal limits on supplemental selenium, what can we do to improve the selenium status of dairy cattle? One potential approach would be to use forages and grains with a higher selenium content. Plants, marine algae and bacteria can convert inorganic selenium from the soil into organic selenoamino acids, like selenomethionine. These organic selenium sources are more available to the animal for absorption and utilization. However, this approach would require a consistent source of high-selenium feedstuffs and monitoring of selenium content would be time-consuming and costly.
One way dairymen have been circumventing the selenium regulations is by strategically using selenium injections. These injectable forms of selenium avoid the problem of poor absorption due to the ruminal environment and are routinely administered to dairy cows during the dry period. This practice has been used for many years as a short-term therapy, but it does very little to improve the long-term selenium status of the animal.
A study that looked at various storage tissues found injectable selenium was no better at improving selenium stores than the no-Se group. In fact, only the group that received the organic selenium source showed significant improvements.
Maas and co-workers found that after an animal was injected with inorganic selenium, Se peaked at five hours postinjection. By 28 days, blood selenium in cows was around 50 ng/ml compared to the 100 ng/ml experienced during the first 24-hour period. So it appears that while injections may work as a short-term therapy, their effectiveness on body stores of selenium is very limited.
Finally, injectable forms of selenium have been known to contribute to spontaneous abortions in the dry cow pen and are responsible for many injection site abscesses in the animals that receive them.
The major advantage of an organic selenium source over inorganic selenium source is its improved absorption and retention in the body. Selenoamino acids incorporated into body proteins provide a reserve of stored selenium when demand for selenium is high, particularly during disease challenge and gestation. Maternal transfer of selenium (through the placenta, colostrum and milk) improves the ability of the calf to survive and thrive.
Summary for production agriculture
The modernization of dairy farms over the last few decades has led to many new stresses being placed on today’s dairy animals. Increased demands for milk production and efficiency are forcing the scientific community to reconsider the nutrient requirements of dairy cattle on a frequent basis. The new NRC made several changes in vitamin and mineral recommendations, and yet federal regulations forced the committee to once again leave selenium at the original 0.3-ppm level.
Recent discoveries and FDA approvals have now opened the door for another way to address our selenium dilemma. Research done over the last several years suggests a more practical approach to improving selenium status is through the supplementation of high-selenium yeast. Yeasts, as part of the plant kingdom, have the ability to convert inorganic selenium into selenoamino acids. These seleno-compounds are what animals need in order to build body reserves of selenium for times of need. PD
References omitted due to space but are available upon request.
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—Excerpts from 2006 Intermountain Nutrition Conference Proceedings
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