Effect of topdress nitrogen rates applied based on growing degree days on winter wheat grain yield

The majority of in-season nutrient management decisions for numerous crops are based on subjective morphological scales. The objective of this study was to establish whether a numerical scale based on growing degree days (GDDs > 0) utilizing weather science, could be used for N management in winter wheat (Triticum aestivum L.). An incomplete factorial within a randomized complete block design (RCBD) was replicated three times, over a period of three growing seasons (2017-2019). The locations were Efaw near Stillwater, OK; and Perkins, OK. A total of 15 treatments were included where three treatments received pre-plant N rates of 0, 90, and 120 kg N ha(-1); and remaining treatments received topdress N of 30, 60, and 90 kg N ha(-1) at 65, 80, 95, and 110 GDDs. Data collection included grain yield, grain protein concentration, and N uptake. Topdress application of N at 80-95 GDDs resulted in improved grain yields at 3 out of 6 site-years. Grain protein concentration increased when N was applied topdress (90 kg N ha(-1)) at 110 GDDs (3 out of 6 site-years). This work shows that an easier-to-use-numerical-scale based on GDDs (80-115) could be utilized for efficient topdress N application in winter wheat.