On September 20, 1997, at 1130 central daylight time, a Mooney M20C airplane, N7045V, sustained substantial damage while landing near Pettigrew, Arkansas. The instrument rated commercial pilot and his passenger were not injured. The airplane was owned and operated by the pilot under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the personal flight which originated from the Northwest Regional Airport, near Roanoke, Texas, at approximately 0810, with the Gaston's Airport, near Lakeview, Arkansas, as its intended destination. A flight plan was not filed for the 303 nautical mile flight. The pilot reported that while en route, the weather deteriorated to broken to overcast ceilings when the flight was near Fort Smith, Arkansas. The pilot further reported that while en route, he noticed the vacuum pressure in the instrument pressure gauge drop below the normal operating range and both gyros started to fail. Approximately 10 minutes after noticing the drop in vacuum pressure, the pressure went to zero resulting in the loss of the artificial horizon, the directional gyro and the wing leveler system. The pilot stated that after losing his gyros, he decided to proceed towards the Huntsville Airport, which was the nearest airport, using the Russelville VOR for navigation. The pilot added that the weather continued to deteriorate and he elected to descend in order to remain in VFR conditions. The airplane descended below the minimum reception altitude for the VOR, resulting in the loss of VOR and DME information. The pilot stated that he elected to execute a precautionary landing in an open pasture. The pilot told the investigator-in-charge (IIC) that the field selected looked level from the air; however, during the landing roll he found that the field had a couple of rises on it which minimized the effectiveness of the wheel brakes. The pilot reported that the airplane skidded into a fence post while traveling at approximately 30 to 40 MPH. The pilot reported that his decision to make the "off-airport" precautionary landing was influenced by the worsening weather, the estimated 30 minutes of fuel remaining, and the loss of the airplane's vacuum driven instruments. The pilot added that the normal range of his airplane is just over four hours, and he had been airborne for about 3.4 hours at the time he elected to land. The FAA inspector confirmed that the left wing spar of the airplane sustained structural damage. A review of the FAA records revealed that the pilot did not get a weather briefing prior to his departure.