When crops are supplied with adequate water and nutrients, crop mass accumulation (g m^-2) is generally proportional to the cumulative amount of intercepted solar radiation (MJ m^-2). The slope of the relationship between crop mass and cumulative intercepted radiation is defined as radiation use efficiency (RUE, g MJ^-1), and can be considered a long-term measure of canopy photosynthesis. The general axiom in crop physiology has been that a greater amount of solar radiation intercepted during the growing season would result in a greater amount of crop mass at crop maturity. If harvest index was not affected by a lengthened cropping cycle, yields would also be expected to increase. Interestingly, for soybean and maize grown in the Midsouthern US, it appears that crop yield reaches an asymptote as the cumulative amount of solar radiation approaches approximately 1200 MJ m^-2. Similar responses have also been observed for soybean grown in Iowa. Under these conditions, it appears that solar radiation is not a limitation to crop yield. The amount of solar radiation intercepted by a crop may be manipulated by changing the planting geometry and crop maturity. The average amount of solar radiation received during the frost-free growing season is approximately 4100 MJ m^-2 (Midsouth) and 3300 MJ m^-2 (Midwest). Multiple short-season cropping systems may be able to more fully utilize the available solar radiation for increased grain yield per year.