The pilot reported that, before conducting a local, aerial application flight, he changed the engine oil. After refueling and loading the airplane with chemicals, the pilot departed. Shortly after takeoff, the windshield became covered with oil, which limited the pilot's visibility. He turned the airplane left toward a grass runway. The pilot did not dump the chemical load, and the airplane had a low airspeed and began to lose altitude. The pilot was not able to attain a climb, so he chose to conduct a forced landing in a cornfield near the end of the runway, during which the wing sustained substantial damage. The pilot reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operation and that the reason the oil covered the windshield was that he had not reinstalled the oil cap after changing the oil. He added that he “had gotten out of …[his] routine” while performing the oil change and that ”complacency” and being in a “hurry were also some factors" in the accident.