Elevating dairy performance with organic selenium
Selenium, like the other trace minerals, is necessary to sustain life and is essential for basic physiological functions in dairy cows. While the daily requirement for trace minerals is small, their importance to dairies has been documented in research. The trace mineral requirements for dairy cows often cannot be met through forage and grain alone. Supplementation in the diet is often needed to bridge the gap between supply and requirement.
Meeting the selenium requirements of dairy cattle
The most recent edition of the Nutrient Requirements of Dairy Cattle defines the selenium requirement for all classes of dairy cows as 0.3 ppm (NASEM, 2021). Selenium is perhaps the most regulated trace mineral in regard to supplementation for dairy cattle. The U.S. Food and Drug Administration (FDA) has set the maximum supplemental level of selenium at 0.3 ppm. Prior to 2003, the only legal forms of selenium in the U.S. were sodium selenite and sodium selenate. In 2003, the FDA approved selenium yeast for dairy cows as a legal form of organic selenium. More recently, in 2020, the FDA also approved the category of selenomethionine hydroxy analogue, a chemically synthetic organic selenium. Selenium supplementation guidelines, as well as approved selenium ingredients, can vary greatly between countries. It is always best practice to first check local regulations when making selenium supplementation decisions.
Organic selenium sources
Plants, marine algae, and bacteria can convert inorganic selenium into organic selenoamino acids like selenomethionine. These organic selenium sources are more available to the animal for absorption and utilization. Yeasts, as part of the plant kingdom, have the ability to convert inorganic selenium into selenoamino acids. A recent white paper (Murphy, 2023) compared form, source and function within the product categories generically defined as “organic selenium.” An important conclusion was that effects on animal health and performance must be assessed for each individual organic selenium product.
The major advantage of an organic 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 is high, particularly during disease challenge and gestation. Maternal transfer of organic selenium through the placenta and colostrum improves the calf’s ability to survive and thrive. Additionally, organic selenium supports reproductive function, udder health and postpartum health.
Focus on reproduction
Research conducted at the University of Florida (Thatcher et al., 2010) compared supplementing 0.3 ppm selenium from either sodium selenite or selenium yeast in 574 dairy cows from 25 days before expected calving date through 80 days of lactation. Results of vaginoscopy scores at 5 and 10 days after calving (Table 1) showed differences based on the source of selenium in the diet. Cows fed selenium yeast had 47.1% clear, 43.4% mucopurulent and 9.3% purulent discharge scores, while cows fed sodium selenite had 35% clear, 47.8% mucopurulent and 17.1% purulent discharge scores. In this study, the diet showed improvements in uterine environment after calving, as the frequency of purulent discharge was reduced while the frequency of clean discharge increased.
Additionally, measurements of immune function may help explain some of the differences reported above. In this instance, the organic selenium yeast improved neutrophil function (innate immunity) at calving in multiparous cows. Neutrophil function was suppressed at calving in primiparous cows but was restored by 7–14 days postpartum with selenium yeast supplementation. The researchers also examined the effects of selenium sources on acquired immunity by measuring the antibody response to an injection of ovalbumin (Figure 1). Antibody concentration was higher in multiparous cows at 21 and 42 days postpartum, while antibody response was not different in primiparous cows.
Selenium source did not impact first-service pregnancy to artificial insemination. However, second-service pregnancy was improved in the selenium yeast diet (17.1% vs. 11.3%). When looking at results for cows that lost an embryo after first service, second service was 22.7% successful for cows supplemented with selenium yeast compared to just 4.2% successful for cows supplemented with sodium selenite. The authors hypothesized that cows in the organic selenium yeast group were better able to reestablish an embryotropic environment at second service following either early or late embryonic losses.
It's in the genes
Other research in cattle reproduction has looked at the effects of selenium source on gene expression. Research at the University of Kentucky compared diets with organic selenium yeast to diets only containing sodium selenite as a selenium source. Cows supplemented with selenium yeast had increased transcripts involved in cholesterol biosynthesis and immune response. 887 transcripts were differentially expressed based on source of supplemental selenium. The upregulation of cholesterol pathways has an important role in increasing the luteal phase concentration of progesterone (Crites et al., 2022a). These same researchers (Crites et al., 2022b) looked at other impacts of selenium source on gene expression regarding endometrial function and development of the conceptus. Cows supplemented with selenium yeast had an increased amount of myostatin gene, which increases glucose secretion into the histotroph that can allow for advanced conceptus development. In this experiment, the conceptus was examined on Day 17 of gestation and was found to be significantly longer in cows supplemented with organic selenium compared to sodium selenite (25.96 vs. 17.45 cm respectively).
A recent dissertation from the University of Guelph (Mion, 2022) also investigated gene expression among cows receiving organic selenium yeast (along with organic sources of other trace minerals) or cows fed sodium selenite (along with inorganic sources of other trace minerals). The author reported that 83 of the differently expressed genes had an expression pattern indicating that the conceptuses from the selenium yeast diet were more advanced in development compared to those from the sodium selenite diet. The gene expression results observed are thought to be required for the coordination of changes in cellular biology that are needed for conceptus elongation, allowing for maternal recognition of pregnancy, implantation and pregnancy survival.
The saying that “form defines function” is both important and true when it comes to supplementing trace minerals to dairy cows. Research around the world continues to prove the impact of organic selenium sources, such as selenium yeast, on reproductive performance. However, not all organic sources of selenium can be expected to perform the same. It’s important to evaluate each option on its own merits to select the best selenium supplement for dairies.