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A method is described for a simple semi-quantitative spot test for glutathione peroxidase in blood of cattle and sheep. The results can be used to place a group of samples from the same herd or flock into one of three blood selenium categories, ie, deficient, low marginal and marginal/adequate.

The reliability of erythrocyte glutathione peroxidase activity as an indicator of selenium status in livestock is discussed. Based on this measurement, a survey is described of the biological selenium status of sheep on each of 329 farms in Britain. Results showed that 47 per cent of these farms were probably unable to provide grazing livestock with sufficient selenium to maintain blood levels greater than 0.075 microgram per ml. Increased selenium deficiency from the increasing use of home grown feeds as a major constituent of livestock rations may be causally related to the increase of white muscle disease and other selenium responsive diseases in Britain.