A single-engine agricultural airplane collided with a cotton trailer while performing an aerial application flight. The 1,296-hour commercial pilot was maneuvering the airplane to fly under power transmission lines when the airplane collided with a parked cotton trailer, lost directional control, and impacted the ground. The operator reported that the pilot was unable to recall the flight due to a concussion sustained during the accident. The operator further stated that a contributing factor was the fact that the pilot was used to flying a 680-horsepower turbine powered airplane and had recently been assigned a 300-horsepower airplane. An eyewitness reported observing the airplane performing the turn to reverse direction before losing sight of the aircraft prior to impact. Additionally, the eyewitness reported that the engine sounded "normal." There were no reported aircraft anomalies prior to the accident.