On July 10, 2009, about 1700 central daylight time, a Grumman/Schweizer G-164B, N6683K, piloted by an airline transport pilot, sustained substantial damage during a forced landing near Beaman, Iowa. The airplane was taking off from a farm airstrip when the loss of engine power occurred. It nosed over during the forced landing to a bean field. The 14 CFR Part 137 aerial application flight was operating in visual meteorological conditions and was not on a flight plan. The pilot was not injured. The local flight was originating at the time the accident occurred. During a telephone conversation with the pilot after the accident, he stated that he experienced a loss of engine power during takeoff, but the engine did not quit. The pilot's written report stated that prior to the accident flight he performed a pre-flight inspection of the agricultural airplane which included checking fuel quantity and water accumulation in the fuel. He noted that the airplane had about 40 gallons of fuel on board and no water was detected in the fuel. The pilot also noted that the remainder of the pre-flight inspection revealed nothing abnormal. He stated that at about 1645, he started the airplane and allowed it to warm up. The engine started normally and oil pressure indications were normal. While the airplane was warming up, the ground crew loaded the hopper with 150 gallons of water and chemicals. The pilot stated that he then performed a run-up of the engine noting that the engine was running smooth and that magneto and carburetor heat checks were good. He verified that the mixture control was set to the full rich position and the propeller was set to low pitch. The pilot reported that the start of the takeoff roll was normal with the engine power set to 35 inches of manifold pressure and 2,300 rpm. He stated that the airplane accelerated normally and he lifted off about 70 miles per hour. He then noticed a loss of engine power and the airplane began to settle back to the runway. He applied full power, but the engine did not respond. The airplane struck a fence at the end of the grass runway and then nosed over in the adjacent agricultural field. The pilot noted that the temperature at the time of the accident was 79 degrees Fahrenheit and the winds were light and variable. The airplane was powered by a Pratt & Whitney R1340 engine. It had accumulated about 300 hours since its last overhaul and about 30 hours since the airplane's last annual inspection. Examination of the airplane subsequent to the accident failed to reveal any reason for the loss of engine power.