lots of farms now have irrigation systems which are basically a long pipe on a central pivot and resting on wheels on the far end. so circular fields make most sense.
"real" crop circles are shapes drawn in fields, not fields made in certain shapes.
the poster knew this, of course, but some people might not.
Actually, these are quite special. They are the result of a project begun in the 1950s to irrigate the high desert. First a large dam was built (creating Navajo Reservoir), then canals to carry the water 40 miles, where they supply center pivot irrigation systems typical of the midwestern U.S. Now, rather than growing nothing but sagebrush & creosote bush, this land grows wheat, potatoes, soybeans, and other crops on the Navajo reservation.
Designs, Farms - Irrigation Circles 
lots of farms now have irrigation systems which are basically a long pipe on a central pivot and resting on wheels on the far end. so circular fields make most sense.
"real" crop circles are shapes drawn in fields, not fields made in certain shapes.
the poster knew this, of course, but some people might not.
anyway, these aren't so special. here's a ton: http://maps.google.com/maps?ll=47.039337,-119.767456&spn=0.514297,0.532150&t=k&hl=en