The available technologies are actually on a continuum, and it depends very much on the crops grown, the moisture content of the grain, and the harvest conditions in each year. If the grain is slightly over-moist, aeration is a very good way to condition that grain and bring it into line with moisture requirements. Grain aeration requires far less energy than propane drying would. That tends to be what we employ on our farm.
If the moisture content is higher, that's where additional natural gas is required. I think that's the particular issue for crops like corn, which are taken off later in the season.