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

Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164-6420

^* Corresponding author (Email: millerbc{at}wsuvml.csc.wsu.edu).

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 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.

Received for publication October 19, 1992.

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