On March 22, 1997, approximately 1130 Pacific standard time, N20113, a Weatherly 620B agricultural aircraft, sustained substantial damage when it nosed over during a precautionary landing near Wasco, Oregon. The commercial pilot, who was engaged in aerial application activities, sustained minor injuries. The flight had departed about ten minutes earlier from a Wasco airstrip. Visual meteorological conditions prevailed at the time of the accident. There was no fire and the airplane was not equipped with an ELT. The pilot stated that his engine began running roughly while he was conducting a spray pass. He pulled up and elected to attempt a precautionary landing in an adjacent field, where he believed he could check out the apparent roughness. During the landing roll, the right wheel dug into the soil and the airplane nosed over when he applied brakes to attempt to stop the ensuing groundloop.