OPTIMIZING YIELD AND GRAIN PROTEIN IN SOFT WHITE WINTER-WHEAT WITH SPLIT NITROGEN APPLICATIONS

Grain protein of soft white winter wheat (Triticum aestivum L.) produc ed in eastern Washington has increased above market-desired levels ove r the past decade, when subnormal precipitation and overfertilization contributed to esca;sire residual soil N levels. A field study was con ducted over four site-years to (i) examine N effects on the yield-prot ein relationship of soft white winter wheat under high soil N conditio ns, (ii) determine if split N al plications can maintain yield and red uce grain protein, and (iii) evaluate midseason grain analysis as a pr edictor of final grain protein. Nitrogen rates ranged from 0 to 140 kg N ha(-1); timing treatments were fall preplant N and spring topdresse d or point-injected N. High yields (>5900 kg ha(-1)) were produced wit hout 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 conjunc tion with shallow N depletion and poor water extraction from deeper so il 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-gears wh ere most soil N was depleted below the 90-cm depth; shallow N depletio n 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. Unde r high residual soil N levels, reduced N rates and split N application s between fall and spring can maintain high yields and reduce grain pr otein,