Effects of Nitrogen Fertilization Rate and Timing on Wheat Grain Yield and Protein in the San Joaquin Valley of California.

This study aimed to demonstrate the effect of N fertilizer management on wheat yield and quality in the San Joaquin Valley. In the 2013 - 2015 growing years, trials were conducted to test the effects of variety, total N applied, and application timing of urea fertilizer. Three varieties, Blanca Grande 515 (hard white), Summit 515 (hard red), and Volante (durum) were fertilized in split applications at the pre-plant, tillering, boot, and flowering stages of wheat development. Total rates of lbs N/acre varied slightly between years, but followed the pattern of residual N only (40-80), low N (130-180), intermediate (225-300), and high (330) with residual soil N always credited. In both years, Blanca Grande grain protein (>13%) was always significantly higher (P=0.02) than that of Volante (~12%), and Summit 515 protein was intermediate. As expected, low rates of fertilizer corresponded with significantly lower yields (P<0.001) for all varieties (< 3 tons/acre), and higher rates corresponded with higher yields (> 3.5 tons/acre). Application timing also had a significant effect (P<0.001) on protein content, where high and low applications at pre-plant yielded the least protein (~10%), and intermediate to high rates balanced among applications yielded the highest protein (~14%).