The inclusion of Se4000 resulted in the highest deposition in milk: 61% increase relative to selenite and outperforming selenised yeast. This also translates to improved Se deposition in colostrum, which can lead to improved passive immunity. This is because L-selenomethionine can be stored in the body in replacement of methionine. As a result, selenium in the form of L-selenomethionine is incorporated and stored in muscle, milk and blood and can be mobilized via selenide to de novo selenocysteine in the liver and subsequent enable selenoprotein synthesis when required. Although selenised yeast has some L-selenomethionine and other organic selenium derivatives, those other derivates cannot be incorporated into animal proteins the same way; less selenomethionine means less deposition and storage reserves.
The more readily available L-selenomethionine (Se4000) allows for improved immune status. In mammals, it was discovered that providing L-selenomethionine significantly lowered expression of Cyclooxygenase-2 (Cox2) and interferon gamma (Ifnγ) relative to selenite, and numerically lower relative to selenised yeast, during normal production. These genes are involved in inflammation when elevated. In the same study during a pathogen (lipopolysaccharide) challenge, mammals that were given selenite as their selenium source had a gene profile that indicated oxidative stress (elevated up-regulation of Txnrd1, Cat, selenogenes SelS and SelN1 before challenge and an immediate down-regulation of Gpx1). Those given an organic source (selenised yeast and Se4000) did not have the same profile. Specifically, Se4000 had an up-regulation of regulatory cytokine interleukin 10 (IL10) which depresses inflammation during times of challenge, redirecting energy away from an inflammatory response to a response important to the producers: performance.
Reducing secondary pathogens
Improving immunity during times of stress can reduce secondary pathogen overgrowth and infections. Proper selenium supplementation can improve the efficiency of leucocytes and antioxidants such as glutathione peroxidase. Without a proper antioxidant defence mechanism, cows can become susceptible to immunosuppression. One study investigated the effect of dietary selenium supplementation on milk composition. Somatic cell counts were significantly lowered by 26% in dairy cows that were given L-selenomethionine for 63 days compared to selenite. Somatic cell counts in the L-selenomethionine treatment were all below 200,000 cells/mL, which is the sensitivity threshold for detection of mastitis. In the same study, free fatty acids were altered in the L-selenomethionine treatment including an increase in conjugated linoleic acid (CLA), rumenic acid. Mammary function can be improved with elevated CLAs by protecting bovine mammary epithelial cells from lipid peroxidation and reducing the levels of reactive oxygen species. Reducing lipid peroxidation can also be evident in dairy products, where L-selenomethionine was shown to reduce lipid oxidation in caciocavallo cheese.
Selenium in ruminants
Selenium absorption tends to be much lower in ruminants relative to non-ruminants due to the ruminal environment creating insoluble forms of selenium. Absorption of inorganic selenium can be as low as 13% in steers and as low as 10–16% in non-lactating and lactating cows. In ruminants, it is especially crucial to provide a selenium source that is readily available.
Heat stress is associated with decreased milk production, increased disease incidence and impaired reproduction. In a commercial dairy study, cows were deficient in selenium (below 65 µg Se/L blood). A total of 30 Friesian cows had diets that included 0.2 ppm Se from selenised yeast replaced by 0.2 ppm Se from Se4000. In two weeks, Se in the blood significantly improved and was no longer deficient (67 µg Se/L). At eight weeks, Se in the blood continued improving (76 µg Se/L) despite elevated ambient temperatures (average 71.6ºF [22ºC]).
