At approximately 1100 Central time the tail-wheel equipped airplane was substantially damaged when it collided with a bale of hay at the Hearne (Texas) Municipal Airport. The Commercial pilot was not injured; the passenger sustained minor injuries. Instrument conditions prevailed; no flight plan had been filed. The pilot later reported that he made a 180-degree turn after landing on Runway 36 and was taxiing to the main ramp at a speed of approximately 30 knots with quartering eight-knot tailwind. When the ramp was approximately 500-600 feet ahead, the pilot decreased power, and the airplane began to veer to the left. The pilot applied right rudder and tapped the left brake, but the airplane continued to veer to the left. The pilot then pulled the stick back to force the tail wheel onto the ground and applied pressure to both brakes but they were not effective. The airplane collided with a bale of hay approximately 200 feet to the left of the runway. Upon impact, the airplane nosed over and came to rest in an inverted position.