Core Ideas Nitrogen availability and fertilization can increase sunflower see yield.Nitrogen fertilization may decrease oil concentration of oilseed sunflower.Nitrogen fertilization increases sunflower lodging risk in windy regions. The N and P recommendations for sunflowers growers in North Dakota have not been changed in 30 yr. Twenty‐two N and P rate experiments were conducted during 2014 and 2015. The objective was to determine the response of seed yield, oil concentration, and lodging to available N and P. In 2014 studies were a randomized complete block split plot with N rate as main plots and P rate as subplots. Nitrogen was applied at rates of 0, 45, 90, 134, 179, and 224 kg N ha−1. Phosphorus was applied to establish P rates of 0, 13, 26, and 39 kg P ha−1. In 2015, the field design included only 0 and 26 kg P ha−1. Experiments were taken to yield and lodging was recorded at harvest. Oil seed sunflower (17 experimental locations) was also analyzed for oil concentration. The N response of sunflower seed yield was quadratic. Increased N rate resulted in lower oil concentration in half of the oilseed experiments. Increased N rate was linearly related to increasing lodging at several sites. Phosphate fertilization had little effect on seed yield, oil concentration, and lodging, despite many of the sites having soil P levels considered ‘low’. Future N rate recommendations should be based on seed yield response with increasing N and oil concentration reduction with increasing N for oilseed sunflower. A limit to maximum N rate should also be considered because of the lodging risk in this wind‐plagued region.

Active-optical (AO) sensors have been used in several crops as a yield-prediction tool for N management, but not in sunflower (Helianthus annuus). The need for in-season N and its rate can be determined through using a yield and AO relationship. By comparing predicted yield from an area of sufficient N to another area of the field, the yield difference multiplied by the N required for the additional yield results in the fertilizer N rate. This study was conducted to determine what parameters, including plant height and plant stand, would be most useful in relating AO sensor readings to sunflower yield. The experiments were conducted in 2015 and 2016 on a total of 20 locations in North Dakota. The experimental design was a split-plot randomized complete block, with six N rates as the main plot treatments and P rate (four in 2015 and two in 2016) as split plot treatment. Since P had no influence on yield, it was ignored in this study. The AO sensors were the GreenSeeker and Holland Crop Circle. The AO sensors were used when the sunflower was at the V6 and V12 growth stages. Manual height and an acoustic height sensor were used on most sites at the time of AO sensor readings. Sensor readings with and without consideration of crop height were subjected to regression analysis with crop yield. The AO sensor readings were related to confection sunflower yield, but not oilseed sunflower yield. Oilseed sunflower yield was most related to crop height.