Optimizing Yield and Grain Protein in Soft White Winter Wheat with Split Nitrogen Applications

Grain protein of soft white winter wheat ( Triticum aestivum L.) produced in eastern Washington has increased above market‐desired levels over the past decade, when subnormal precipitation and overfertilization contributed to excessive residual soil N levels. A field study was conducted over four site‐years to (i) examine N effects on the yield‐protein relationship of soft white winter wheat under high soil N conditions, (ii) determine if split N applications can maintain yield and reduce grain protein, and (iii) evaluate midseason grain analysis as a predictor of final grain protein. Nitrogen rates ranged from 0 to 140 kg N ha −1 ; timing treatments were fall preplant N and spring topdressed or point‐injected N. High yields (>5900 kg ha −1 ) were produced without fertilizer N, and yield responses to N ranged from 0 to 22%. Fall N < 56 kg N ha −1 increased yield in only one site‐year; yields were reduced due to excess N fertilization in another site‐year in conjunction with shallow N depletion and poor water extraction from deeper soil layers. In two of four site‐years, yield increased with a 50% fall‐50% spring point‐injected N compared with 100% fall application at 84 kg total N ha −1 . Protein >100 g kg −1 was produced in site‐years where most soil N was depleted below the 90‐cm depth; shallow N depletion was associated with lower protein. Grain N concentration at maturity was highly correlated with grain N concentration at the late milk and soft dough stages. Preharvest predictions of final grain protein may be useful in segregating grain at harvest for marketing purposes. Under high residual soil N levels, reduced N rates and split N applications between fall and spring can maintain high yields and reduce grain protein.