Sowing date (SD) is sensitive to regional climate characteristics; thus, it is critical to systematically examine the effects of SD on crop yields for various temperature regimes. We performed a sensitivity study of SD for maize in the southwestern U.S. using the regionally extended version of the Agricultural Production Systems sIMulator (APSIM) model. The model was run utilizing North American Regional Reanalysis at a 32 km resolution from 1991 to 2011, with an irrigation threshold at 95% of the soil water-holding capacity. Two types of SD optimizations maximizing yield potential (Yp), varying spatially or interannually, revealed that the optimal SD varies according to regional climate characteristics and depends on the base temperature climatology during the growing season. For cool regions at high elevations (e.g., northern California and northern Nevada) and in coastal areas, earlier sowing results in higher Yp, allowing longer growing seasons. In these regions, yearly varying of SD to reduce the negative effects of springtime cold events can also enhance Yp significantly. In low-elevation warm regions (e.g., southern Central Valley, southern California, and southwestern Arizona), the length of the growing season rarely impacts Yp, and early planting is crucial to avoid adverse impacts of extremely hot conditions in the summer. For transitional regions (e.g., the southern Great Basin in Nevada and the Colorado River basin in Arizona), high Yp can be obtained in a short growing season due to the optimal temperature range of the growing season. Thus, for the transitional regions, SD optimization does not have much impact on Yp
