What levels of selenium are essentials?

Soil

Acidic basalt, granite, peat and volcanic soils combined with a rainfall above 500mm are typical of a region or soil type prone to selenium deficiency.

Indicative levels of selenium deficiency

<500 parts per billion – marginal deficiency
<300 parts per billion – severe deficiency

Pasture

While soil levels are the main determinant for selenium in pasture, low levels can be accentuated by clover dominant pastures and a light suppression due to applied sulphur. Rapid pasture growth in spring can also dilute the pasture selenium levels.

There is a move to lifting the suggested pasture level to 40-50ppb. Vitamin E levels in pasture are important but these are generally sufficient in New Zealand.

Blood

There are a number of methods of determining selenium levels from blood samples. Glutathione Peroxidase (GSH-px or GPx) is a measure of a selenium containing enzyme produced by the liver and reflects selenium intake about two months prior. Serum levels reflect current selenium intake while whole blood levels are between these two time periods. Alternatively liver samples can be taken from animals going to slaughter. Serum selenium levels from blood samples taken from the same animal(s) at two month intervals are the best indicator of whether selenium levels are rising or falling.

Toxicity

Selenium is only required in trace amounts. Too much selenium can be as damaging as too little. Chronic toxicity could be expected if stock levels were consistently 50-1— times the adequate levels shown in the tables opposite.

The tables below show levels set by MAF Qual in trials investigating selenium responsiveness. However as more work comes to light the levels may be amended. Generally there is a trend to seek higher levels.

 

Sheep:

Test Deficient Marginal Adequate Units

GSH-px level
(Glutathione
Peroxidase)

< 20 (ill thrift)
< 2
< 3.5
2-4
> 35
4-25
Units/g HB
kU/L @25
Whole Blood
Selenium
< 10
< 112
10-20
112-225
> 20
> 250
ug Se/L (ppb)
noml/L
Plasma
Selenium
< 7
< 50
7-11
50-100
> 11
> 100
ug Se/L (ppb)
noml/L
Liver
Selenium
< 0.2
250
0.2-0.5
250-450
> 0.5
> 450
mg/kg dry (ppm)
nmol/kg

 

Cattle:

Test Deficient Marginal Adequate Units

GSH-px level
(Glutathione
Peroxidase)

< 10
< 0.5
< 20
0.5-2.0
> 20
2.0
Units/g HB
kU/L @25

Whole Blood
Selenium

< 10
< 130
10-15
130-250
> 20
> 250
ug Se/L (ppb)
noml/L

Plasma
Selenium

< 8
< 85
8-12
85-140
> 12
> 140
ug Se/L (ppb)
noml/L

Liver
Selenium

< 0.2
< 600
0.2-0.5
600-850
> 0.5
> 850
mg/kg dry (ppm)
nmol/kg

 

What action should you take?

Monitor

The best way to be sure of the selenium levels in your stock is to monitor these levels several times a year. A once a year analysis will give you the levels at a single point, but not indicate whether levels are rising or declining. Remember selenium is required year round not only at specific times.

A selenium application method which provides optimal levels over long periods is therefore essential, both in terms of convenience and efficacy.

The choices of selenium supplementation

1. Direct Animal Routes

Drenches – drenches have been the traditional method used to supplement selenium. Resultant selenium blood levels are low and short lived (3-6 weeks. Relying on a selenised drench programme will generally not provide year round selenium supplementation or selenium at the crucial time from birth to weaning.

Vaccines – some vaccines contain selenium. This selenium is of a readily soluble form and is at a relatively low level. Blood levels are only elevated for a short period.

Injections – Both soluble and insoluble forms are available. The soluble forms give a quick but short 3-6 week duration. Slow release injections must be administered under the skin in the neck (not in the muscle), selenium supplementation is sustained.

2. Pasture Applied

Fast release granules – this granule is based on sodium selenate and gives an initial excessive pasture peak. This peak lasts for 1 to 3 months and subsequent levels return to pre application levels after 5 – 8 months. Stock brought onto the pasture after this period may need alternative forms of supplementation until the next application. High rainfall, irrigation, thin free draining souls and some peat soils are likely to exacerbate this problem.

Selcote Ultra Granules – no other form of supplementation should be necessary as grazing stock should receive a continuous supply of selenium naturally through the pasture. Selcote Ultra can be applied direct to the pasture (or forage crops) or conveniently added to your annual fertiliser requirements – remember to ask for Selcote Ultra, not just selenium.