Grain protein as a post-harvest index of nitrogen status for winter wheat in the northern Great Plains

The use of grain protein as a post-harvest index of N fertility status has been promoted for spring wheat (Triticum aestivium L.) through the establishment of critical levels for segregating wheat into N deficient vs. N sufficient classes. The objectives of this study were to evaluate this concept for winter wheat in the northern Great Plains; and to estimate the added N requirements necessary to achieve maximum yield when protein concentrations fall below the critical level. A field study consisting of three water regimes, four cultivars, and five fertilizer N levels was conducted near Havre, MT. A consistent relationship between relative yield and grain protein was found and a critical protein concentration of 121 mg g -1 was defined using Cate-Nelson R 2 statistics. Protein concentrations below the critical level were associated with yield losses from N deficiency (79% frequency), while protein concentrations ≥ the critical level were associated with N sufficiency (93% frequency). Under conditions of mode rate N deficiency (68–99% of maximum), protein concentration could be used to estimate the amount of additional N that would have been needed to achieve maximum yields. This is accomplished by first calculating the difference between the critical and actual protein concentration (expressed in mg g -1 protein). This protein deficit is then multplied by a fertilizer N equivalent that varied from 20 to 38 kg N ha -1 (according to the precipitation environment) for each 10 mg g -1 rise in protein desired. Key words: N sufficiency, N deficiency, critical protein concentration, plant available N