The pilot/owner of the Piper PA-18-135, accompanied by his wife, was taking off from runway 12, a turf/grass strip, at Marion, Montana. Winds were light and out of the southeast at the time. The pilot reported that during the takeoff roll he lost directional control of the aircraft, which then departed the runway crossing a ditch and collapsing the main landing gear. The pilot reported that there was no mechanical malfunction with the aircraft during the event.