ACCORDING TO THE PILOT, THE AIRPLANE OPERATED NORMALLY UNTIL AFTER ARRIVING AT THE FIELD TO BE SPRAYED. HE STATED THAT DURING THE FOURTH SWATH RUN, THE AIRPLANE DID NOT FEEL AS 'SOLID' AS IT HAD EARLIER. HE ELECTED TO MAKE A PRECAUTIONARY LANDING IN A NEARBY FIELD. DURING THE LANDING ROLL, THE MAIN LANDING GEAR COLLAPSED. AN EXAMINATION OF THE ENGINE FAILED TO DISCLOSE A MECHANICAL PROBLEM. DURING A TEST RUN, THE ENGINE PRODUCED RATED POWER.