spring freezes (Carter, 1995;Kunkel and Hollinger, 1995), and excessive precipitation and flooding during Weather and climate have had major influences on crop production the growing season (Kunkel et al., 1994). in the Upper Great Lakes states of Michigan, Minnesota, and Wiscon-Analyses of the impact of weather and climate on sin during the past century. However, isolation of the impact of weather is made difficult by the confounding effects of technological agriculture for extended time periods have been freimprovements in agriculture, which have resulted in significant grain quently constrained by the lack of quality long-term yield increases. The objective of this study was to identify climatologiclimatological data series and the limited number of cal impacts involved with the production of three crops commonly experimental treatment combinations available from grown in the region-alfalfa (Medicago sativa L.), maize (Zea mays field experiments. In addition, it is difficult to isolate L.), and soybean [Glycine max (L.) Merr.]-without the influence of the impact of weather due to the confounding effects technology, and trends of relevant agroclimatological variables during of technological improvements in agriculture (e.g., imthe period 1895-1996. The models DAFOSYM, CERES-Maize, and proved varieties, increased rates of fertilization), which SOYGRO models were used to simulate crop growth, development, have resulted in significant yield increases during the and yield of the three crops, respectively. Regionally, low precipitation past century (Thompson, 1986). An alternative strategy and moisture stress were chief limitations to simulated crop yields. is the use of crop simulation models that are based on Simulated maize and soybean yield series were found to increase with time an average of 11.4 kg ha Ϫ1 yr Ϫ1 and 4.9 kg ha Ϫ1 yr Ϫ1 , respectively, the underlying physiological processes governing plant across the study sites during the study period. These increases were growth and development. Such models, if properly caliassociated with average study period increases in total seasonal precipbrated and tested, allow a user to easily investigate the itation of 0.4 mm yr Ϫ1 and decreased total seasonal potential evapoeffects of individual input variables by holding all others transpiration of 0.2 mm yr Ϫ1 . No consistent trends were found for constant and provide a more convenient, less expensive alfalfa. The simulated yield results support previous research identitool than long-term field research in the evaluation of fying a period of benign climate, which favored crop production in crop response to environmental and management facthe region from 1954 to 1973, and was preceded and followed by tors (Angus, 1991). Crop simulation models can also be periods of relatively greater yield variability.used to investigate multiple factors and their interactions at various hierarchical levels, including farm, regional, and national scales.