Influence of Late-Season Foliar Nitrogen Applications on Yield and Grain Nitrogen in Winter Wheat

Increasing grain protein in new higher-yielding cereal grains has recently received added attention due to protein premiums paid to farmers. Hard red winter wheat (Triticum aestivum L.) studies were conducted at two locations in Oklahoma in 1997–1998, 1998–1999, and 1999–2000 to evaluate the effects of late-season foliar N applications on grain yield, total grain N, straw yield, and total straw N. Foliar applications of N were made at two different times (pre- and postflowering) using urea ammonium nitrate (UAN) at rates of 0, 11, 22, 34, and 45 kg N ha−1. Ammonium sulfate [(NH4)2SO4] was also applied at a single rate of 22 kg N ha−1 both pre- and postflowering. A significant linear increase in total grain N was observed for postflowering applications using UAN in five of six site-years. In four out of the six site-years, a significant linear increase was observed for preflowering applications of UAN. No consistent increases or decreases from foliar N applications were observed for grain yield, straw yield, or straw N. Over years and locations, UAN applied preflowering and postflowering at 34 kg N ha−1 increased total grain N over that of the check (no foliar N applied) by 2.7 and 2.4 g kg−1, respectively. Late-season foliar N applications before or immediately following flowering may significantly enhance grain N content and, thus, percent protein in winter wheat.