Weather and climate have had major influences on crop production in the Upper Great Lakes states of Michigan, Minnesota, and Wisconsin during the past century. However, isolation of the impact of weather is made difficult by the confounding effects of technological improvements in agriculture, which have resulted in significant grain yield increases. The objective of this study was to identify climatological impacts involved with the production of three crops commonly grown in the region-alfalfa (Medicago, sativa L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]-without the influence of technology, and trends of relevant agroclimatological variables during the period 1895-1996. The models DAFOSYM, CERES-Maize, and SOYGRO models were used to simulate crop growth, development, and yield of the three crops, respectively. Regionally, low precipitation and moisture stress were chief limitations to simulated crop yields. Simulated maize and soybean yield series were found to increase with time an average of 11.4 kg ha(-1) yr(-1) and 4.9 kg ha(-1) yr(-1), respectively, across the study sites during the study period. These increases were associated,,vith average study period increases in total seasonal precipitation of 0.4 mm yr(-1) and decreased total seasonal potential evapotranspiration of 0.2 mm yr(-1). No consistent trends were found for alfalfa. The simulated yield results support previous research identifying a period of benign climate, which favored crop production in the region from 1954 to 1973, and was preceded and followed by periods of relatively greater yield variability.
