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Effect of topdress N rate applied based on growing degree days on winter wheat (Triticum aestivum L.) grain yield

Dhillon, Jagmandeep Singh
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 (GDD> 0) utilizing weather science, could be used for nitrogen (N) management in winter wheat. An incomplete factorial within a randomized complete block design was replicated three times, over a period of three growing seasons (2017 to 2019). The locations were Efaw near to Stillwater, OK and Perkins, OK. A total of 15 treatments were included where three treatments received preplant 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 GDD's. Data collection included normalized difference vegetation index (NDVI) sensor readings, grain yield, grain protein concentration, and N uptake.
Peak NDVI values were recorded between 90 and 120 GDD's. Topdress application of N at 80 to 95 GDD's resulted in improved grain yields for three of the site years compared to preplant N applications. Grain protein concentration increased when N was applied topdress (90 kg N ha-1) at 110 GDD's (3 out of 6 sites years).