Intraspecific Variation in the External and Internal Copper Requirements of Subterranean Clover 1

For diagnosis of Cu deficiency in pastures based on subterranean clover ( Trifolium subterraneum L.) and for the management of the Cu nutrition of animals grazing them, it is important to know the extent of intraspecific variation of subterranean clover in internal Cu requirement, response to soil Cu supply, and ability to accumulate Cu in herbage. In the present study, three cultivars (‘Clare,’ ‘Trikkala’, and ‘Seaton Park’) representative of three major subgroups of subterranean clover were grown for 70 days (D70) on a Cu deficient soil supplied with varying rates of Cu fertilizer up to 800 µ g Cu/3 kg soil (Cu 800 ). The relationships among Cu supply, yield response, amount of Cu in herbage, and Cu concentrations in young leaves were examined. At optimum Cu supply, all three cultivars produced similar amounts of dry matter. At Cu 0 on D70, Clare, Trikkala, and Seaton Park produced 9, 26, and 30%, respectively of their maximum shoot dry matter which they reached at Cu 800 , Cu 400 , and Cu 200 , respectively. At all equivalent levels of Cu supply, the shoots of Clare had lower Cu contents than did those of Trikkala and Seaton Park: the effect was relatively most marked at Cu 0 where shoots of Clare had only one‐half and one‐third of the Cu in the shoots of Trikkala and Seaton Park. It is suggested that Clare had a poorer ability to obtain Cu from the soil in this experiment than did Trikkala and Seaton Park and this accounted for its poorer growth at low Cu and its greater response to higher levels of applied Cu. By contrast with their differing external Cu requirements, the three cultivars appeared to have very similar internal Cu requirements for maximum dry matter production. Estimates of critical concentrations of Cu in the youngest open leaf at just adequate Cu supply gave values of about 3 µ g Cu/g dry matter for all cultivars.