Selenium
Selenium is a necessary component of glutathione peroxidase, whose known function is to minimise oxidative tissue damage. Challenge is greatest at times of greatest intake of polyunsaturated fatty acids, for example when grazing rapidly growing spring grass.
Effects
Effects of deficiency can be subclinical, possibly confined to depression of growth, immune response and phagocytic activity. The most specific clinical effect is white muscle disease - sudden and severe myonecrosis, usually bilaterally symmetrical, affecting the muscles of the limbs, diaphragm and often the heart, causing sudden death. These effects, which are direct effects of oxidative tissue damage, can be precipitated by exercise.
Selenium deficiency is also associated with a wide range of conditions where the biochemical mechanisms are not understood. These are abortion, weak or stillborn calves, retained placenta and infertility. In sheep, early embryonic death and resorption is a common feature.
Occurrence
As well as having serious effects, selenium inadequacy is widespread in the US. Low pasture levels are found at about the same order of frequency as for copper.
There are also known areas where selenium toxicity can occur, usually where there is a basin or other geological feature causing a local concentration of leached salts. These areas are generally circumscribed and well recognised.
As for copper, selenium intake in grazing animals varies with pasture composition, species dominance, stage and rate of growth. Young, rapidly growing pasture is most often associated with problems. This makes it hard to predict selenium intake on a geological basis, since pasture factors, including soil ingestion (a source of selenium) are often more important, and vary within the same property and from month to month. Knowledge of the history of a particular property is valuable, but problems can be precipitated by for example land improvement, with fertiliser application and re-seeding.
Diagnosis
There is good placental transfer, and the cow will deplete her own liver copper reserves while foetal levels increase. The ability of milk-fed calves to absorb copper is much greater than in ruminating calves.
Functions
Gross and histological post mortem findings for white muscle disease are highly characteristic. Red cell glutathione peroxidase is reliable (15 to 25 units per mg of hemoglobin per minute is marginal for cattle). However, the red cell GPX level only reflects the status when those cells were forming. Once cells are circulating, their GPX content is not thought to change. This test is therefore retrospective - it tells you what the status was about 6 weeks previously. In view of the rapidity with which pasture conditions can change, this needs to be fully considered in assessing results.
Serum selenium gives a good index of the status on the day of sampling. However, this test is not universally available.
Published requirements (NRC) are 0.05 to 0.3 ppm selenium of total feed dry matter. Inclusion of sodium selenite or selenate is an easy matter where cattle are fed but, as always, grazing is the problem.
With free access, it is difficult to give enough to supply the backward feeders without risking toxicity in the greedier animals.
Since there is some persistence of GPX in tissues, a degree of control is possible by oral drenching, about 2 weeks before service and before parturition, particularly in sheep. However, this does not fit well with stock handling. In some countries, a slow release injection is available, although injection site abscesses can be a problem.
Animax manufacture selenium and cobalt boluses, Tracesure Cattle/Calf and Tracesure Sheep/Lamb, which provide selenium and cobalt by slow release for 5 - 6 months
Products
The following products feature this element:
