Customizing Nitrogen Fertilizer and Seeding Rates in Soft White Winter Wheat.

New cultivars of soft white winter wheat (Triticum aestivum L.) are being continually developed and released. These cultivars often have higher yields, superior disease resistance, and improved end-use quality compared to older cultivars. Current fertilizer recommendations guides are a useful resource for estimating nitrogen (N) rates. However, reinvestigation of crop response to fertilizer-N application is needed to ensure optimal recommendations as new cultivars are released, and the cost of inputs and crop returns are constantly changing. Field trials were established at three locations in northern Idaho to examine six modern cultivars or advanced lines of soft white winter wheat in combination with fertilizer-N rates and seeding rates. These cultivars and advanced lines include: UI-WSU Huffman, IDN01-10704A, IDN02-29001A, SY-Ovation, LCS Artdeco, and LCS Drive. Six nitrogen rates (0, 25, 33, 42, 50 and 58 g N kg^-1 expected yield) and three seeding rates (148 to 247 seeds per m²) were included in the study. Seeding rate had little influence on performance or yield. In 2015 and 2016, the economic optimum nitrogen rate (EONR) varied from 32 to 48 g N kg^-1 of expected yield depending on the variety. However, higher yields were obtained with higher rates of nitrogen and given a higher wheat price and lower nitrogen price, the EONR could change. Averaged across locations and years, the advanced line IDN01-10704A had the greatest yield, while LCS Drive yielded the least. From a regression of seed yield and nitrogen rate across all sites and years, IDN01-10704 had the greatest N efficiency producing 37.7 kg of grain for each kg of fertilizer-N, while the efficiency was lower for other cultivars (28.0 to 31.3 kg grain for each kg of fertilizer-N).