Need selenium?

It is easy to look at a label, check that the product contains selenium and never give it a second thought, but questions might prompt a look at the source of selenium used for supplementation.

What is selenium?

Selenium (Se) is a powerful biological antioxidant essential to all mammals. Selenium is present within at least 25 proteins -- called selenoproteins. These proteins protect against oxidative damage, have a role in thyroid function (thus affecting growth rates), influence reproductive success and are integral to (and enhance) the immune response system, to name a few key functions. Without dietary selenium these functions are impaired or cannot occur.

What are the main consequences of selenium deficiency in cattle?

In cattle, Se deficiency can lead to white muscle disease, cystic ovaries and metritis, retained placenta, early foetal abortions, weak still-born calves, infertility affecting oestrus, ovulation, embryo fertilisation and development, and low sperm motility.

Are there natural sources of selenium?

Soil is the main natural source of selenium. Soil Se levels directly affect plant selenium concentrations. NZ’s soils are notoriously low in Se which results in low levels being expressed in crops and grass, leading to reduced intakes, whether as direct grazing or via locally grown grains/crops. 

As plant material alone is not an adequate source of selenium for calves and cattle in NZ, it must be supplemented in a bioavailable form. Fishmeal is usually a good source of selenium but this fishmeal is expensive and not usually fed to milking cows. When selenium is present in plants -- and meat or eggs for that matter -- it is present in the naturally occurring organic forms elenomethionine and selenocysteine.

What selenium sources are available on the market in NZ?

Selenium sources are either inorganic or organic. Sodium selenite and sodium selenate are inorganic forms of selenium that are cheap, water soluble and widely available. These sources are typically mixed with other trace minerals and dosed through drinking water or added to feed, or added as the sole Se source to mineral injections. Barium selenite is also an inorganic selenium source that is typically administered as an injection. 

Organic selenium sources available in NZ are selenoyeasts and hydroxy-selenomethionine (hydroxy-Se-Met). 

Selenoyeasts are produced by growing yeasts on an inorganic selenium medium, allowing the yeast to integrate Se into the cell. 

Is organic selenium is better than inorganic selenium?

Scientific evidence shows the superior bioavailability of organic selenium sources compared to inorganic sources (across many different species). 

In ruminants however, it appears that absorption of inorganic selenium is diminished as rumen microbes reduce (change) most of the dietary inorganic selenium into a form that cannot be absorbed or utilised (inorganic selenide forms).

It is estimated that only 25-30% of selenium from sodium selenite is bioavailable to ruminants. On the other hand, many studies have shown that feeding organic Se to ruminants increases Se levels in blood, colostrum and milk. 

• Jennifer Ross is technical services officer, BEC Feed Solutions NZ.