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Selenium
Written by John Turner Consultancy for Agrifert
Selenium is an essential ultra-trace element essential for the nutrition of all animals. Care must be taken when applying selenium as this element is also potentially very toxic.
Symptoms of dificiency:
The classic symptom of selenium deficiency is 'white muscle disease' of lambs, the msucle most affected is the heart. This results in heart failure or paralysis of the hind limbs and resulting in a high mortality rate for new born animals.
Slight deficiency is associated with ill-thrift and scouring, female animals have reduced fertility and cows that get into calf typically have increased incidence of 'retained placenta' at calving time. Retained placenta can result in minor infections that also reduces fertility.
Different classes of livestock have different requirements for selenium:
As for most essential nutrents,selenium deficiency affects young animals more than fully grown livestock. Adequate selenium is also impotant for cows and ewes from before mating to the end of the gestation period.
Factors that influence the selenium content of pastures:
There is a reasonably good correlation between the soil selenium test level, the plant tissue selenium status and animal status. One problem with selenium treatment of soil has been the fact that sodium selenate (the most common selenium compound used) is very water soluble and has a short effective life in the soil unless incorporated into a slow release product such as 'Selcote'.
Where the soil pH is low, plant uptake of selenium will be reduced and soil applied selenium products become 'less effective'. In parts of the world where selenium toxicity is an issue, it is refered to as 'alkali disease' due to increased uptake of selenium from soil minerals when pH is high.
Sulphur at high levels in the diet will reduce selenium absorption by grazing animals, brassical leaf tissue often has high sulphur levels.
Level of Selenium required in pasture or feed:
The level of selenium that is considered adequate in pasture and animal feeds is 0.10 mg/kg however some consultants recommend levels up to 0.30 mg/kg for dairy cows. The threshold of toxicity depends on dietary factors that influence selenium absorption however 5.0 mg/kg is likely to be toxic.
To quantify this amount of selenium, 0.10 mg/kg is the same as 0.1 gram per 1000 kg dry feed, a very small amount.
Options for Selenium treatment:
Addition of selenium prills to base fertileser blends is the stadard approach to providing selenium for grazing animals. Usually the rate is 1 kg of a 1% selenium product per hectare per year, manufacturers of these products (such as Nufarm) produce information about release patterns and pasture Se levels.
Selenium has also been added to animal drench products however the rate added may not be effective where stock are 'deficient'. Vets have often advised farmers not to use selenised drench if they have applied prills to the soil however as there is a 30 to 50 fold margin of safety between deficiency and toxicity,this is very cautionus advise.
Injection products are marketed, particularly for young lambs and calves which contains selenium and vitamin B12 (cobalt metabolite). This is intended as a dose to reduce mortality and ill-thrift due to sub-clinical deficiency of both elements.
Treatment of drinking water and inclusion of selenium in foliar treatment products are possible treatment options however caution is essential to avoid the possibility of selenium poisoning. A foliar treatment product to provide 0.30 mg/kg 2000 kg Dm requires 0.6 grams of selenium per hectare.