Critical levels and nitrogen fertilizer equivalencies for grain protein in winter wheat

Authors: LONG, DANIEL - Collaborator; Reardon, Catherine - Kate; Adams, Curtis

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2023
Publication Date: 11/28/2023
Citation: Long, D., Reardon, C.L., Adams, C.B. 2023. Critical levels and nitrogen fertilizer equivalencies for grain protein in winter wheat. Agronomy Journal. 116(1):339-348. https://doi.org/10.1002/agj2.21499.
DOI: https://doi.org/10.1002/agj2.21499

Interpretive Summary: Much of the soft white winter wheat grown in the inland Pacific Northwest is exported to Asian markets where low grain protein is desirable. Targeting low grain protein concentration in wheat is challenged by conditions of drought and heat stress that produce wheat of reduced quality and yield. Precision nitrogen (N) application methods have been developed for dryland wheat to target grain protein using N fertilizer recommendations that are based on replacing the amount of N removed in the grain of the previous crop. To further develop this method for soft white winter wheat, this study determined the amount of N (or N equivalent) required to increase (or decrease) grain protein concentration (GPC) by one unit. This relationship was determined for four soft white winter wheat (SWW, Triticum aestivum L.) cultivars grown under a wide range of precipitation. The study was conducted for three years in Oregon, USA near the towns of Lexington (225 mm or 9 inches of mean annual precipitation) and Adams (450 mm, ~18 inches) in Oregon, USA. Precipitation gradients and different N fertilizer rates were both used to create a range of variability in grain yield and protein for these cultivars. The critical protein concentration, which indicates whether N nutrition is adequate for yield, was determined across all cultivars as 117.5 g kg-1 (or 11.8 %). This concentration could be used as a broad-based indicator of N sufficiency. Analysis of the amount of available soil N and GPC revealed that 4 to 7 kg N ha-1 (4.5 to 7.8 lb N ac-1) is required to change grain protein by one unit (1 g kg-1, 0.1%) in lower precipitation areas of the region where SWW is under water stress during grain filling. This information is useful for precision fertilization strategies intended to that target a specific grain protein concentration across a field.

Technical Abstract: Precision nitrogen(N) application methods have been developed for dryland wheat that utilize site-specific measurements of grain protein concentration(GPC) to determine N fertilizer recommendations for the next season. The objectives of this study were to determine the critical protein level and N equivalent to a unit change in GPC from relationships between GPC, and grain yield or plant-available N in soft white winter wheat (SWW, Triticum aestivum L.), and assess the consistency of these relationships across SWWcultivars grown under a wide range of precipitation. A 3-year study was undertaken near two sites: Lexington (225 mm of mean annual precipitation) and Adams (450 mm) in Oregon. Differences in precipitation and N fertilization rates between sites were used to induce variability in grain yield and GPC of four cultivars. A critical protein concentration of 117.5 g kg-1 was defined by Cate–Nelson analysis of scatter plots of relative yield versus GPC. Critical protein among cultivars ranged between 105 and 118 g kg-1 suggesting that 117.5 g kg-1 might be used as a general indicator of N sufficiency. Slopes of the regression of available N on GPC were consistent across cultivars and revealed that 4.2–8.4 kg N ha-1 is equivalent to a unit change protein (1 g kg-1) in lower precipitation areas of the region where SWW is under water stress during grain filling. This information is useful in calculating the N to apply from the GPC in the previous season to meet crop requirements in the next season.