Effect of sulfur application on selenium content of subterranean clover plants grown at different levels of selenium supply

Abstract

To assess the feasibility of adding selenium to pastures likely to be associated with White Muscle Disease and related disorders in stock, plants of subterranean clover (Trifolium subterraneum L.) were raised in pots and supplied with rates of selenium (as selenate) and sulfur (as sulfate) in factorial combinations. On the soil used, increasing rates of selenium progressively depressed growth of clover and increased its selenium concentration; sulfur stimulated growth up to an addition of about 16 yg S/g and depressed the selenium concentration only when selenium was added. More sulfur was required to depress the selenium concentration to a threshold level, which was characteristic of a particular selenium application rate, at high initial selenium levels than at low. Clover growth was reduced when the sulfur to selenium ratio in the tops was less than 50: 1. Application rates greater than 0.025 ¦g Se/g soil (= 40 g/ha) produced young clover plants with excessive levels of selenium for consumption by stock. The safe rate for application to a mixed pasture is likely to be considerably less than that value. Field experiments confirmed that appreciable diminution of the selenium concentration in pasture as a result of sulfur addition occurred only when selenium was in plentiful supply. Grasses were much more affected than clovers. It is concluded that the negative interaction demonstrated between selenium and sulfur is of no practical significance when each is added at rates appropriate to the treatment of low sulfur-low selenium pastures.