The pilot initiated a cruise descent toward the destination airport. Shortly after reducing power, the engine began to run rough, according to the pilot, and then stopped running as the airplane reached the pilot's predetermined lower altitude. The airplane performed a forced landing in a field of milo crops approximately 1,000 feet north of the airport boundary. Subsequent investigation revealed the pilot did not apply carburetor heat before reducing the airplane's power at the start of the descent. An on scene investigation revealed the airplane had approximately half its normal fuel load. The airplane was operating on auto gas without an STC. The pilot confirmed this and said he was not aware of the requirement to have an auto gas STC. The airplane was out of its required 14 CFR Part 91 annual inspection by three years. The nearest Federal Aviation Administration certified weather reporting station was 57 nautical miles south of the accident site. The temperature/dew point for that location was 65 degrees Fahrenheit and 49 degrees Fahrenheit respectively. These conditions are conducive for the formation of carburetor icing.