Irrigated hard winter wheat response to fall, spring, and late season applied nitrogen

High yielding irrigated hard red winter wheat (HRW, Triticum aestivum L.) is frequently low in protein and marketed at a discount. Protein is affected by available nitrogen (N) but few published reports relate to N fertilization practices for producing high yielding irrigated hard red winter wheat with acceptable protein in arid environments. A three year study (1985–1987) was conducted (1) to determine the influence on irrigated HRW yield and protein of fall preplant, spring topdress, and late N applied, and (2) to evaluate flag leaf nitrogen concentrations (FLN) for scheduling late season N. Early season N treatments for vegetative growth consisted of an untreated control (0 N), and 168 kg urea N ha−1 either broadcast preplant and incorporated in the fall, topdressed in early spring, or both (336 kg N ha−1 total). Applying late season N at heading or flowering (0 or 56 kg urea N ha−1) was evaluated with all earlier season N treatments. The N applied for vegetative growth increased yield from 3.6 to 4.1 Mg ha−1 and protein from 15 to 47 g kg−1. The timing of N application (fall versus spring), without late season N, did not affect grain yield but protein was 8–13 g kg−1 higher with spring topdressed N. Excessive N (336 kg N ha−1) during vegetative growth consistently reduced grain yield of non-lodged wheat due to lighter kernels. Protein increases with late season N were highly dependent on the earlier season N rate and timing. Late season N was essential for increasing protein to 120 g kg−1 without incurring yield losses from excessive N during vegetative growth. FLN was directly related to protein, and inversely related to the late season N induced protein increase. However, the protein relationships to FLN differed markedly among years. Normalized FLN values, dividing all values for a year by the highest value in that year, greatly reduced the year effects. Both high yields and acceptable protein of irrigated HRW are possible with sufficient N during vegetative growth for yield and late season N for protein enhancement.