At 12:50 CDT, an Air Tractor AT-502 ag plane was damaged following a loss of control on takeoff. The pilot was not injured. The pilot said he attempted to take off from a private dirt airstrip which is also used as an access road. The pilot said the hopper was filled with a load of rice seed to be dispersed on a nearby field. The pilot said that he had begun his takeoff roll and had lifted the tail wheel when he observed a white pickup entering the airstrip/road to observe his flight. The driver of the pickup truck, the farmer whose fields were being seeded, had observed previous takeoffs. On this flight, however, a heavy load required a longer ground roll. As the airplane closed to within a few feet of the pickup truck, the pilot attempted to break ground prematurely to clear the truck, and the airplane stalled.