An Analysis of Temporal and Regional Variation in Wheat Yields

The important meteorological and agronomic factors contributing to variation in grain yields over large areas are generally known, but their relative impact on temporal and regional variation has not been generally quantified. In any given year, one or more factors may be given undue credit for high or low yields. This investigation determined the contribution of certain meteorological and agronomic variables to variation in grain yields of fall and spring-planted wheat (Triticum aestivum L. and T. durum Desf.) on a large-area basis. We previously developed a winter wheat (WW) and a spring wheat (SW) model to predict gram yields over large areas without the benefit of direct measurement of plant characteristics. The derived relations of meteorological events, applied N, and certain soil characters to yield were considered to be universally applicable temporally and globally. In this study, we applied the models to different years (1955 through 1976) and regions in the U.S. Great Plains and Corn Belt. For WW in semiarid regions of the U.S. Great Plains, soil moisture deficiencies were more important than high temperatures or excessive precipitation as a yield determinant but, for SW, high temperatures were more important than soil moisture-related variables. During 1955 to 1976 weather conditions for WW yields were poorest during 1956 and best during 1973. Weather conditions for SW yields were poorest during 1961 and best during 1958. Increases in WW yields between 1955 and 1976 attributed to applied N varied from 1.0 quintals/ha in semiarid zones to 2.9 quintals/ha in the more humid Corn Belt. Increases attributed to higher-yielding cultivars ranged from 7% in the Southern Plains to 25% in the Corn Belt for WW and from 15% in the western portion to 25% in the eastern portion for SW. Yields of WW over the 22-year period averaged 9.8 quintals/ha higher in the Corn Belt than in the Southern Plains. That difference was attributed to more favorable weather, heavier applications of N, higher-yielding cultivars, and better soils in the Corn Belt. The difference would have been larger were it not for irrigation in the Southern Plains. Higher yields in the eastern SW region were attributed to more favorable weather, better soils, and higher-yielding cultivars.