The pilot of the aerial application helicopter had sprayed hundreds of acres earlier in the day and was positioning the helicopter at another farm site for spraying the following day. He said he selected a landing site next to a grain trailer to remain clear of on-going farm operations. The pilot stated he began his approach at the end of a 7-hour duty day, hovered adjacent to the trailer, and focused his attention on the position of the spray booms as he descended over the landing spot. As he lowered the collective control to complete the landing, the main rotor blades struck the top and side of the grain trailer which resulted in substantial damage to the helicopter’s main rotor blades and tailboom. The pilot reported that there were no mechanical deficiencies with the helicopter that would have precluded normal operation. In the NTSB Form 6120.1 Pilot/Operator Aircraft Accident/Incident Report the pilot offered several safety recommendations to prevent similar accidents from happening in the future. Among them, he suggested a "higher level" of aeronautical decision making, improved fatigue awareness, better obstacle awareness, and maintenance of landing area scan and avoidance of "fixation" on any one item or obstacle.