Developing successful strategies to ensure future increases in crop yield depends, in part, on a better understanding of the basis for past increases. To this end, I compared historical yield trends of two dissimilar crops—corn (Zea mays L.), a high yielding C4 grass, and soybean [Glycine max (L.) Merr.], a moderate yielding C3 legume—in high (Iowa, Illinois, Indiana) and low (Kentucky, Missouri, Tennessee) yield environments. Average state corn yield changed very little from 1866 to 1930, an era of low-input agriculture, but it increased rapidly from 1950 to 2005 in all states as the low-input system gave way to a high-input system that utilized commercial hybrids, manufactured N fertilizer, herbicides, and higher plant populations. Soybean yield, first available in 1924, increased steadily from the beginning until 2005. Corn yield increased faster than soybean yield in the early decades of the high-input era (mean growth rates of corn were 3.3 vs. 1.5% per year for soybean with a slight advantage for both crops in high yield states) in all six states, but for the rest of the era (up to 40 yr), corn and soybean yields grew at nearly the same rate (1.8% per year for corn, 1.4% per year for soybean with a slight advantage for the low-yield states). Thus, the efforts to improve the plant (plant breeding) and production environment (better management practices) had essentially the same affect on these two very dissimilar crops.