Selenium (Se) is one of the essential trace elements required in poultry and animal nutrition. It is proved to play a vital role in the antioxidant defense mechanism of the body. Selenium is the cofactor for the enzyme glutathione peroxidase which catalyzes the removal of peroxides (oxidative reaction of free radicals) thus important in the prevention of oxidative stress. It has a role in improving reproductive health too.
Its deficiency in chickens results in exudative diathesis (hemorrhagic disease), nutritional muscular dystrophy, gizzard myopathy (turkeys), dystrophy of the heart (turkeys), and degeneration of pancreas apart from reproductive issues such as impaired hatchability, fertility, reduced viability of sperms, etc.
SOURCES OF SELENIUM
Two major sources of selenium (Se) in poultry are :
- Inorganic Se in the form of sodium selenite or selenate.
- Organic selenium in the form of selenomethionine (SeMet)
In natural feed ingredients like grains, soya & oilseeds, selenium is available in the form of selenomethionine (around 50% of the total available Se). In contrast, commercial Se sources constitute sodium selenite and selenate (Surai, 2006). Mahmoud and Edens (2003) reported that the glutathione peroxidase activities in the liver of broilers significantly increased when fed with organic selenium under heat stress conditions. Their experiments and results even stated that organic selenium has higher bioavailability and antioxidant properties compared to inorganic selenium.
In 1974, FDA approved the supplementation of selenium in the poultry and swine feed in the form of selenite or selenate (Surai, 2006). Since then it has been used as a source of selenium and various disadvantages have been documented. Sodium selenite in feed interacts with other nutrients such as ascorbic acid and is reduced to the elemental form which is biologically unavailable in the digestive tract. If selenite is dispersed in feeds with high water activity it is converted to the seleniferous acid (volatile form) and can be lost as a vapor. Pro-oxidant effects of selenite possess detrimental effects on the gut and vacuolar degeneration of epithelial cells in the crypts of the duodenum, mononuclear infiltration in the ileum (Atria et al., 2010). Sodium selenite is hardly transferred to an egg resulting in the limited antioxidant defense against oxidative stress for a growing embryo (Surai, 2006). It cannot be stored in the body as a reserve to combat the stress and most of the selenium consumed is simply excreted.
Considering the above disadvantages due to the use of inorganic selenium, the concept of organic selenium was developed in 2000 and a range of organic Se products appeared in the market (Surai, 2006).
SeMet naturally available in the feed ingredients is advantageous over the traditional selenite. SeMet transferred to the eggs provides a stronger antioxidant defense to the developing embryo and improves the hatchability and viability of newly hatched chicks in the breeder nutrition (Fisinin et al., 2008; Surai and Fusion, 2014). Chick embryo antioxidant capacity improved and better expressed under oxidative heat stress conditions when fed in the form of selenomethionine rather than sodium selenite through deposition from maternal diet. Under heat stress conditions SeMet showed a higher value in reducing the oxidative damage than sodium selenite. Glutathione peroxidase activity in the liver of chick embryos increased when organic selenium deposition was noted through maternal diet thus increasing their anti-oxidant activity in heat stress conditions (Xiao et al., 2015).
In layer diets organic selenium is associated with higher efficiency of transfer to eggs and improved internal egg quality (Haugh Unit) through the activation of a selenoprotein, methionine sulfoxide reductase B (Msr B) required for the prevention of protein oxidation and maintaining the water holding capacity of the albumen (Jlali et al., 2013; Tufarelli et al., 2016). Selenium also has a positive effect on the eggshell quality, breaking strength as it improves the organic matrix formation (Surai, 2006). Organic Selenium increases the transfer of Se to the muscles and also building up reserves in the body which have a positive impact on the immunity, gut health, meat quality and protects against stressors (Surai and Fisinin, 2014).