Sunola response to nitrogen fertilization

The response of sunola (Helianthus annuus L. 'AC Sierra') to nitrogen (N) fertilization under conventional and direct seeding tillage systems was examined in field experiments at sites in the semiarid Dark Brown and subhumid Black soil climatic zones over a 2-yr period of normal to above-normal growing season precipitation. Urea-N fertilizer was banded at the time of seeding at varying rates to a maximum of 150 kg N ha(-1). Tillage treatment did not influence sunola response to N fertilization. Sunola seed yield responded greater to fertilization at the Black soil site compared with the Dark Brown soil site in both years. Sunola harvest indices were comparable across sites and years, ranging from 0.10 to 0.18. Seed oil concentration averaged about 4% higher at the Black compared with the Dark Brown soil site, and responded to fertilization differently between locations. Sunola water use efficiency was markedly higher in 1994 than in 1993 at both sites, primarily due to lower water consumption by the crop. Fertilizer recommendations can be based on a desired yield goal or marginal rate of return. A minimum acceptable marginal rate of return of 1, 1.5 and 2 times the cost of the fertilizer, indicated that fertilizer N plus soil nitrate-N levels of greater than 94, 66 and 38 kg N ha(-1) respectively, were uneconomical at Scott; respective rates at Melfort were 183, 174 and 154 kg N ha(-1). However, fertilizer rates in excess of 100 kg N ha(-1) in the Black soil climatic zone can result in potentially high levels of residual nitrate-N in the soil profile, and thus have greater potential for N leaching and denitrification losses.