To look at transpiration one must compare the crops in question with what they are replacing. In the case of the Midwest of the USA, the corn and soy are replacing forest, and tallgrass prairie. Both of these ecosystems have longer growing seasons and greater total leaf area than the crops, and so probably total transpiration for the year is less with the crops. In areas with extensive irrigation schemes, such as the Central Valley of CA where I now live, the issue is probably reversed, but there doesn't seem to have been any significant increase in
local rainfall. I did read a research paper which said this extra moisture is carried by winds and leads to increased rainfall elsewhere in the Southwest of the US.