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.