Why is selenium important in cattle diets?
Selenium forms part of a number of enzymes and other proteins in animal tissues. In particular it is a component of the enzyme glutathione peroxidase (GSH-PX). This enzyme inhibits and destroys naturally occurring peroxides that cause cell damage. It acts in conjunction with vitamin E to protect cell membranes including cell walls. This protection is particularly important in muscle cells that work hard and consume large quantities of energy and oxygen.
Selenium is found in the soil and taken up by plants at different levels depending on plant species, fertiliser application and rainfall. Cattle consume selenium with the plants they eat. It is stored for a short period in the body, mainly in the liver, so a continual dietary supply of selenium ensures the best possible production.
What are the signs of selenium deficiency?
When there is a deficiency of selenium, harmful free radicals are generated. These damage muscle tissues of the heart and limbs (skeletal muscles). This disorder is called nutritional muscular dystrophy (NMD) or white muscle disease. Selenium is also very important in maintaining a healthy immune system so deficient cattle may be more susceptible to some common infectious diseases.
Clinical signs of selenium deficiency in WA include:
- suboptimal milk production
- suboptimal fertility in adult cattle
- retained foetal membranes (RFM)
- premature, weak calves, perinatal death and abortions.
In places with severe selenium deficiency, the following clinical signs may also occur:
- in young cattle: ill-thrift, poor growth rates, chronic diarrhoea and retention of winter coats
- acute nutritional muscular dystrophy in young calves:
- sudden collapse or death of calves within 2–3 days of birth
- high mortality rates.
- subacute nutritional muscular dystrophy in older calves (1-4 months):
- stiff-legged gait
- weakness and unable to stand or walk.
Which animals are most at risk?
Selenium deficiency most commonly occurs in young calves and calving cows, but is also seen in adult cattle.
Where is selenium deficiency most common in Western Australia?
Cattle are predisposed to selenium deficiency when grazed on:
- pastures grown on selenium-deficient soils (such as acid soils receiving more than 410 millimetres annual rainfall)
- lush, rapidly growing pasture
- legume-dominant pasture
- paddocks that have received heavy or long-term sulphur-containing or superphosphate fertiliser applications.
In WA, selenium deficiency is seen mainly in the higher rainfall areas of the south-west, particularly along the coast.
How can a veterinarian help?
Early diagnosis of disease and treatment are essential to minimise production and stock losses. A veterinarian can help diagnose selenium deficiency by collecting blood or post-mortem samples for laboratory analysis. They can also provide help on the most appropriate selenium supplementation program for your herd and property.
How is selenium deficiency diagnosed?
A combination of clinical signs suggestive of selenium deficiency, geographical area and laboratory samples can help with a diagnosis of selenium deficiency.
Recent selenium status of cattle can be assessed by testing blood samples from some of the herd or liver samples from an affected animal. These are the most accurate ways to assess selenium status.
Soil samples are not suitable for determining risk of selenium deficiency in animals as the total soil selenium level includes elemental selenium and selenite, both of which are relatively unavailable to pasture plants and selenium uptake by plants is inconsistent.
Pasture or feed samples
Stock have a higher requirement for selenium for health than plants so plant tissue testing is not very useful. Assessing selenium levels in total mixed rations for dairy cattle may be done as part of a herd health program.
How to prevent selenium deficiency
There are three main ways in which selenium can be supplemented. Advice from your local veterinarian and agronomist should be sought as each case and farming system are different.
Selenium applied to pasture
Selenium concentration of pasture is increased following the application of 10 grams selenium per hectare as either sodium selenate or barium selenate.
Application of selenium in fertiliser has become widely accepted as a cost-effective way of meeting selenium needs of high stocking rate dairy, sheep and beef farmers. Frequency of application is influenced by soil type, dry matter yield of forage growth, rainfall or irrigation and the selenium requirements of the different stock classes.
Incorporation into the soil, uptake by the plant and intake by the animal may take several seasons, with slow-release barium selenate offering a more prolonged release of selenium compared with sodium selenate.
Selenium supplied as drenches, injections or pour-ons
Drenching, injections (via subcutaneous injection) and pour-on methods of selenium supplementation are available. Injections containing barium selenate as their main active ingredient have been found to provide cattle with up to 12 months’ effective selenium levels in blood.
Selenised lick blocks and loose licks
Selenised lick blocks and loose licks are also available but, due to variable consumption of these products by animals within a group, blocks and licks cannot ensure adequate levels of selenium are delivered.
Intra-ruminal selenium pellets
Intra-ruminal selenium pellets are an effective method to address selenium deficiency in cattle.
Selenium is one of the most toxic of the trace elements. Cases of toxicity in cattle can be due to overdosing of selenium supplements, providing more than one selenium-containing supplement at the same time when animals already have high levels from their environment.
Clinical signs of selenium toxicity include:
- breathing and respiratory distress
- anorexia and weight loss
- fast heart beat
- increased incidence of urination
- sudden death.
Daily intakes of greater than 0.25 milligrams selenium per kilogram, a single oral dose of greater than 10mg Se/kg or a single injection of greater than 1.2mg Se/kg bodyweight are toxic for cattle.
Always obtain advice from your local veterinarian before deciding on a supplementation program of selenium for your livestock.
For more information on selenium deficiency, contact your local veterinarian or a Department of Primary Industries and Regional Development, Western Australia (DPIRD) field veterinary officer.
The Livestock Biosecurity program contacts webpage lists DPIRD field veterinary officers in each region.