Nitrogen and Phosphorus Recalibration for Modern Varieties of Sunflowers for the Northern Great Plains.

Sunflower producers in the United States are presently directed to nitrogen (N) and phosphorus (P) recommendations that originate from research performed in the late 1960s and early 1970s. Since development of these first nutrient recommendations, production practices of crop rotations and conservation tillage, as well as technologies of defensive breeding and crop protection products have advanced within sunflower production. Updating the N and P recommendations for sunflower is vital for improving nutrient use efficiencies, determining economic thresholds, and maximizing current applied research. In 2014, one year out of a two-year field study was completed with 11 dryland sunflower field sites in North Dakota and three dryland sunflower field sites in South Dakota for recalibrating N and P recommendations for modern varieties of sunflower. Each field study site included N fertilizer rates of 0, 45, 90, 135, 180, and 225 kg ha^-1 and P fertilizer rates of 0, 34, 67, and 101 kg ha^-1. Yield and oil measurements were analyzed using least significant difference (LSD) and quadratic polynomial correlation with N rates. Yield response to N was observed at nine of 14 sites, with the greatest sunflower yield average across nine sites of 1650 kg ha^-1 observed at the 225 kg ha^-1 N rate. Yield response to P was observed at two sites in 2014, the 101 kg ha^-1 P rate increasing yield on average by 270 kg ha^-1 at these two sites. Sunflower oil content responded to N and P- at four and three sites, respectively, with the 101 kg ha^-1 P rate yielding the greatest percent increase in oil, and the 45 kg ha^-1 N rate providing the greatest percent increase in oil over the check.