Nitrogen Nutrition and Yield Relations of Nugaines Winter Wheat

A nitrogen rate study was undertaken to determine the potential utility of the critical nutrient concept as a guide to N fertilization of winter wheat (Triticum aestivum L.), as part of a more extensive soil fertility investigation in western Oregon. This work on wheat was prompted by the need for a suitable diagnostic technique for determining more precisely the optimum levels of available N. Only limited information exists on the applicability of the critical nutrient concept or on critical levels of plant N in winter wheat, particularly the semidwarf types. Top grain yield was reached after N application of 140 kg/ha, but there were no significant yield differences over the range from 84 to 224 kg/ha. Plant NO₃-N values in leaves and shoots at tillering, jointing, and boot stages of growth were increased significantly by the highest N increment of 224 kg/ha compared to the next lower N rate. In contrast to grain yield, the highest values for total N percentage in plant shoots at all the later growth stages and dry matter yield at the soft dough stage were obtained with the highest rate of applied N, which also produced the highest values for plant NO₃-N. Semidwarf wheat utilized the high N rate more efficiently for dry matter production than for grain yield, and high N fertility also increased grain protein percentage. A graph of grain yield vs NO₃-N concentration in plant leaves at jointing showed a definite peak in grain yield at approximately 500 ppm NO₃-N in the leaves. Additional applied N very readily increased leaf NO₃-N considerably above 500 ppm and caused a slight, insignificant drop in grain yield. These results suggest the applicability of the critical nutrient concept to semidwarf wheat, and provide justification for additional work to establish more precisely the critical level for NO₃-N in wheat.