Plant

Selenium is not considered a plant essential nutrient, however recent research suggests it may have anti-oxidant functions.

While crops do not necessarily require selenium, having adequate levels helps ensure  animals consume sufficient concentrations of selenium in their diet.  

In forage, selenium levels of 0.25-0.35 mg/kg of dry matter are considered optimum.

Animal

Selenium is a component of selenoproteins - a range of proteins involved in immunity, thyroid function, sperm function and amino acids. 

Selenium works together with vitamin E as an antioxidant which helps protect animal tissues and red blood cells. 

A lack of selenium can result in poor growth, reduced fertility, mastitis, retained cleanings and white muscle disease.

White muscle disease is a nutritional muscular dystrophy where the fibers do not function resulting in muscular weakness. Effected animals have a rapid heartbeat and respiration rate, weak legs, muscle stiffness and tremors. 

If left untreated, death ultimately occurs as a result of cardiac failure. 

Soil

Around 90% of Irish soils are deficient in selenium.

Selenium is taken up by plants in the form of selenate (SeO₄^2-). Its negative charge means that it is unable to bind to negatively charged clay particles and so is at risk of leaching.

Selenium is also present in the soil as selenite (SeO₃^-) which is not available for plant uptake. In the presence of air, selenite can be converted to selenate by aerobic bacteria, which can be used by the plant.

High levels of soil sulphur can reduce selenium availability.

Since selenium is not plant essential, plants are unlikely to show visual signs of selenium deficiency. The biggest sign of selenium deficiency is likely to be associated with livestock grazing selenium deficient pastures or forage. 

Deficiency symptoms in livestock include...

• Ill-thrift
• Retained placenta
• Muscular dystrophy (white muscle disease)
• Reduced weight gain
• Mastitus
• Suppressed fertility

Where is the risk of deficiency highest?

• Light, sandy soils
• Areas of high rainfall
• Low organic matter soils

Testing

A Broad Spectrum (BS) soil analysis can help determine the quantity of selenium which is likely to be available for crop uptake and can be used to tailor fertiliser plans and to help prevent deficiencies within the season.

Tissue testing is also useful to determine the selenium concentration of the plant mid-season in order to compare against optimum levels. Although laboratory results may be available too late to correct the deficiency in the current crop, they can be useful for decisions on selenium use for future crops.

Blood tests on livestock can help indicate whether there is a selenium deficiency within the animals diet.

Selenate (SeO₄^2-) and selenite (SeO₃^-) are not held on soil particles. Once the soil is fully wetted, selenate may leach into field drains or subsurface aquifers as drainage water moves through the soil.

The amount of winter rainfall has an important influence on the amount of selenate and selenite leached from the soil profile.

Right Product

Products which contain multiple sources of selenium (selenate and selenite) provide selenium which is immediately available to the plant, as well as sustained release selenium which will be made available through the course of the growing season. 

Right Time

Best applied in the spring/summer, little and often to maximise crop uptake and to prevent nutrient loss.

Right Place

By supplementing the soil with selenium, rather than directly supplementing the animal, farmers can build soil selenium reserves and reduce the need for selenium supplementation in the future. 

Right Rate

Around 10mg/ha of selenium is usually sufficient to raise forage selenium levels to 0.25-0.35mg/kg dry matter.