Shortly after the acft took off with 4 skydivers on board, its eng seized at an alt of about 300 to 400 ft. The plt selected the best available area to land which was a field of tall corn on rolling terrain. During the emerg landing, the acft nosed over. Later, the eng was disassembled & aprx 1 qt of finely ground ferreous material was found deposited in the lower oil pan. The main bearings were found scored & discolored, & the main bearing shells showed evidence of overheating. The connecting rods were also discolored from heat. The screen chamber of the oil pump was found full of ferrous metal which shut off the oil supply to the oil pump gears. The faa maint inspector who examined the eng stated that the ferrous material found in the eng & bottom pan was not from the eng. The source of the material was not determined.