During a crop spraying operation, the pilot noticed oil on the windscreen. Because the amount of oil was increasing he decided to proceed to a private airstrip to make a precautionary landing. As the aircraft climbed away from the cotton field, smoke was observed issuing from the engine cowl. The pilot advised his markers he would attempt an emergency landing at Cudgewa airstrip. Shortly after there was a loud noise as the propeller separated from the engine. The aircraft was landed in a wheat field but overturned during the landing roll due to the soft surface. The hopper contained 150 litres of chemical at the time of the accident, some of which splashed over the pilot. The propeller was found to have separated from the engine drive shaft after the retaining nut which secured the propeller to the shaft had come off. The bolt designed to secure the retaining nut on the propeller shaft was missing for reasons which were not determined.