Selenium is a micronutrient that is an essential component of many enzymes that have a wide range of functions, particularly as an anti-oxidant protecting against cell damage. In youngstock, selenium is most commonly associated with white muscle disease, but in adult cattle, selenium deficiency has been linked with a wide range of problems.
The level of selenium in pasture is dependent on the level of selenium in the soil. In many areas of the UK, the concentration of soil selenium means that the concentration of selenium in the grass is marginal to low (Less than 0.1mg/kg). Unsupplemented cattle at pasture, such as late lactation or dry cows and cycling heifers are therefore more likely to show signs of selenium deficiency than housed cattle on a balanced mineral ration.
Clinical signs
Many diseases are associated with selenium deficiency. However, it is important to remember :
- That these diseases are caused by selenium deficiency is nowhere near as conclusive as the link between white muscle disease and selenium deficiency
- These diseases are associated with many other factors than selenium deficiency. Concentrating on a mineral deficiency may result in the actual cause being missed.
Diseases linked with selenium deficiency include:
- Retained fetal membranes
- Cystic ovaries
- Anoestrus, and poor oestrus behaviour
- Early and late embryo death
- Mastitis and increased somatic cell counts.
- Some farms will show only one sign, others several
Diagnosis
- Selenium deficiency cannot be diagnosed on clinical signs alone. A proper veterinary investigation is essential.
- Blood samples: Selenium can be measured in blood, but a selenium enzyme found in the red blood cells (glutathione peroxidase (GSH-Px)) is more commonly used. Animals are usually defined as being selenium deficient when GSH-Px activity is < 30 U/ml of red blood cells
- Other samples: Kidney and liver samples can also be used to estimate selenium status, but are more difficult to collect than blood. Milk selenium concentration can be a useful indicator of selenium status, but measurement is significantly more expensive than blood GSH-Px.
- Too often a diagnosis of selenium deficiency is made on response to treatment with selenium
- Many of the problems associated with selenium deficiency develop over a long period of time; separation of the placenta is a process that occurs over a period of up to three weeks, while cystic ovaries are often diagnosed 40+ days after the abnormal ovulation that resulted in the cyst. If there is a dietary change over this period, the selenium status can change, so that animals that were selenium deficient appear normal and vice-versa.
- The animals sampled must be sampled before the problem began. If it is a retained fetal membrane problem sample dry cows not fresh calvers, and if cows are cystic or not-pregnant, examine them before service not after pregnancy diagnosis.
- To confirm selenium deficiency, it is best to analyse the selenium content of the diet and, if grazing, the soil selenium concentration.
Treatment
The first priority must be to treat the symptoms seen, e.g. progesterone treatment for cysts and intramammary antibiotics for mastitis. Selenium supplementation is a long-term solution. If supplementation is required, a proper preventative programme must be instigated
Prevention
The control and prevention of selenium deficiency depends upon increasing the supply of selenium. Vitamin E is commonly supplemented with selenium, as there is a significant interaction between the two. Supplementation can occur via :
- Injection: Strategic injection of selenium/ vitamin E at specific timepoints, such as 20 days before calving for the prevention of retained fetal membranes, can be beneficial particularly in cattle that are not receiving supplements. Ask your vet for advice
- Dietary supplementation: The dietary requirement for ruminants is 0.1mg/kg of selenium, which is easily and cheaply attained with the inclusion of selenium in feed supplies or salt and mineral mixes. Selenium can be toxic, therefore all feed should be analysed before supplementation begins.
- Pasture topdressing: The application of sodium selenate to pasture (at a rate of 10g/ha) can be used as an economic alternative to individual dosing.