D. Tomic - Department of Field and Vegetable Crops, Faculty of Agronomy, University of Kragujevac, Cara Dušana ۳۴, +۳۸۱۳۲۰۰۰ Čačak, Serbia.
V. Stevovic - Department of Field and Vegetable Crops, Faculty of Agronomy, University of Kragujevac, Cara Dušana ۳۴, +۳۸۱۳۲۰۰۰ Čačak, Serbia.
D. Djurovic - Department of Field and Vegetable Crops, Faculty of Agronomy, University of Kragujevac, Cara Dušana ۳۴, +۳۸۱۳۲۰۰۰ Čačak, Serbia.
R. Stanisavljevic - Laboratory for Seed, Institute for Plant Protection and Environment, Teodora Drajzera ۹, Belgrade, Serbia.

A field experiment involving red clover varieties K-۳۹, K-۱۷, Una and Viola was established on an acidic (pHH۲O ۴.۸) soil to evaluate the effects of foliar application of cobalt (a beneficial nutrient for efficient nitrogen fixation) on seed yield, and on seed yield components. The foliar spray of the crop was carried out using cobalt nitrate [Co(NO۳)۲] with the treatments: one application at the intensive growth phase during the first growth, while two others during the second growth within the second year of cultivation. Seed yield and yield components were recorded from the second growth within the second year of the study. Regardless of foliar cobalt application, the varieties produced a significantly higher seed yield in ۲۰۱۱, when the rainfall received from the onset of flowering until seed maturation was recorded as lower than that in ۲۰۱۰. The foliar treatment with cobalt was in general accompanied by a positive effect on seed yield and seed yield components in all the varieties. As compared with control, a significant increase in seed yield, in the cobalt applied treatment, was obtained only in Viola, mostly due to the significant increase in flower number i.e. seed number per inflorescence. The more favorable response of Viola to foliar cobalt application may have been attributed to a greater percentage of foliage cover during the treatment, as compared with the other varieties. This suggests that foliar cobalt treatment in future studies should be performed at the early stages of development i.e. during intensive growth throughout the first and second cuts, in order to stimulate nodulation and have greater nitrogen fixation in a needed timely manner.

Cobalt, Foliar spray, Moisture regime, Red clover, Seed yield, Yield components