Prior to the flight, the pilot performed a preflight inspection, but he did not inspect the rudder pedals and rudder cable attachments. During takeoff, approximately 50 feet above ground level, the pilot felt something unusual with the left rudder pedal. The airplane immediately veered to the right, and the pilot initiated a climb in an attempt to troubleshoot the problem. Unable to determine what happened, the pilot maneuvered the airplane back to the airport. The airplane was difficult to control to the left and he determined he had to make a landing on the grass adjacent to the runway. During the landing, the airplane impacted a drainage ditch. The airplane sustained substantial damage to the firewall. Postaccident examination of the airplane revealed the bolt/nut hardware that connected the left rudder cable to the rudder pedal had separated, and the cable became disconnected from the pedal. The nut, which is to be secured by a cotter pin, backed off of the bolt. The bolt and nut were found within the airplane. The experimental amateur-built airplane had accumulated approximately 19 hours of flight time, at the time of the accident.