The most irrigated crop in the US is grass.

As those who have lawns know, maintaining an ideal lawn can require large amounts of water in certain regions of the US. During the winter, grass grows more dormant and requires less watering and mowing.

During the summer, when grass can dry out and turn brown, people are inclined to use more water to recover the fresh green appearance. Because of this, it is estimated that up to 70% of residential water is used for mowing lawns. That makes it more irrigated than any other crop in the US. 

In 2005, Nasa found over 30 million acres of irrigated lawn in the US, which is THREE TIMES the area of irrigated corn. Funny how much we value a nice lawn, isn't it? 

(Source)