Selenium

Overview

Trace minerals are essential to an animal for normal health and productivity. They act as the keys that assist in the utilisation of energy and protein within the body. As such they are important for normal growth, meat, wool and milk production, fertility and immunity. Trace minerals are required constantly but at certain times such during the growth of a young animal or during pregnancy and lactation or when experiencing increased stress for whatever reason, the animal’s demand can increase.

Deficiencies can present either as overt clinical disease or, more commonly, subclinically with less obvious signs but still significant productivity losses. If clinical disease is present this often the “tip of the iceberg” and many more animals are deficient and not showing obvious signs.

Deficiencies of selenium, cobalt and copper are common in ruminants in many parts of Australia. We will cover selenium deficiency in the next section.

Importance of selenium

Selenium has a number of important functions. It acts as an antioxidant to protect tissues from cell damage as is also involved in normal healthy metabolism and function of the immune system.

Many areas of Australia are predisposed to selenium deficiency such as regions with lighter soil types, acidic basalt or granite or sandy soils, higher rainfall regions (450 – 500mm per year), clover dominant pastures and areas of heavy or long term fertiliser application e.g. high superphosphate application

Pasture selenium content is also diluted during periods of rapid pasture growth therefore there can be a seasonal pattern to deficiency. Furthermore, selenium levels are dependent on pasture species and shallower rooting species (white clover or perennial ryegrass) may have lower levels than deeper rooted species (lucerne)

Periods such as weaning, pregnancy or lactation will increase the demand for selenium and sometimes tip animals which might be marginally deficient over into clinical disease

Symptoms

Signs of selenium deficiency may be non-specific and look like any other cause of general ill-thrift. Animals may show reduced wool and/or meat production, poor immunity (shown by increased levels of mastitis or faecal worm egg counts for example), reduced reproductive efficiency, weak or stillborn calves or retained foetal membranes in cattle.

One presentation of selenium deficiency which is strongly indicative of a deficiency is White Muscle Disease (WMD). WMD results in the destruction of normal skeletal and cardiac muscle. Animals can show muscle pain, lameness, reluctance to move, stiffness, proppy gait and even death especially if heart muscle is involved. On necropsy, the affected red muscle tissue appears pale and is sometimes described as like fish flesh in appearance.

Treatment

The first step in addressing a selenium deficiency problem is determining whether the animals are deficient or not. This is important not only to decide if treatment is necessary but also because too much selenium can be toxic and supplementing animals with adequate levels is a waste of money and potentially harmful. Blood tests are simple and accurate and results can be received in a few days. The test used involves measuring blood levels of an enzyme called glutathione peroxidase (GSHPx) activity. If the results show a deficiency is present then treatment is advisable.

Treatment can be carried out in a variety of ways. Topdressing pasture with selenium or directly supplementing stock orally via loose licks or pellets or by subcutaneous injection are amongst the options available. It should be noted that while many vaccines and worm drenches contain selenium, the levels are generally low and only provide a short lived top up of selenium for a few weeks or months. If selenium deficiency is diagnosed, a treatment targeting it specifically in the longer term is the best approach. 

Providing selenium via injection is popular because it is simple and reliable and provides extended selenium levels for 12 to 18 months depending on the species. Treatment is advisable during winter, spring and summer. Timing treatment to occur at weaning, pre-joining or pre-lambing will meet periods of increased demand. Treating before breeding will also benefit the foetus in the mother’s uterus ensuring they have enough selenium early in life.

Selovin LA

Selovin LA is a long acting selenium injection for cattle and sheep. It contains selenium (50mg/mL) as barium selenate which is a slower release form of selenium. This makes it more convenient because fewer treatments are required and also safer. It treats and prevents selenium deficiency in cattle for up to 12 months and sheep for up to 18 months. There are no withholding periods for Selovin LA.