Abstract: | Abstract Nitrogen use efficiency and response of sunflower (Helianthus annuus L.) to timing and rate of surface banded N was characterized in a split‐plot 4x2 factorial experiment. Nitrogen rates (main plots) were 0, 34, 67, and 134 kg ha‐1 at Mississippi State and 0, 45, 90 and 180 kg ha‐1 at Brooksville, MS. Nitrogen, applied as NH4NO3, was surface banded either at planting or at the four leaf stage (subplot). Seed yield was significantly influenced by rate of N application at both locations. Seed yield showed a quadratic response at Mississippi State and a Mitscherlich‐type response at Brooksville. Maximum seed yields of 2606 and 2380 kg ha‐1 were obtained at the respective sites. Sunflower responded to N fertilizer application when inorganic N content of the soil to 60 cm depth at planting was less than 50 kg ha‐1. Nitrogen efficiency was influenced by rate and timing of application, exhibiting exponential declines with increasing N rates. Fertilizer losses at the highest rates of applied N were 19 and 52% at Mississippi State and Brooksville, respectively. Clay‐fixed NH^+ accounted for 26% of the applied N fertilizer loss at Brooksville. Nitrogen fertilizer efficiency and recommendations for sunflower could be improved if initial soil inorganic N is taken into account. |