The commercial pilot reported that he had been applying rice seed to a field throughout the day and was returning to land on a dirt airstrip. During the approach, the airplane encountered a downdraft and the right main landing gear impacted the top of a seed dispensing truck that was positioned near the approach end of the airstrip. The pilot continued the landing and upon touchdown, the right main landing gear collapsed, the airplane veered to the right, crossed over a ditch and came to rest in a field. The airplane sustained substantial damage to the right main landing gear attaching point to the fuselage. The pilot reported that there were no mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot landed to the north and reported the wind was gusting about 8 to 15 knots.