On Sunday, May 16, 1993, at approximately 1934 central daylight time, an Aero Commander 680, N6244D, was substantially damaged when it collided with an animal and subsequently ran off the side of the runway during landing at a private airstrip in Marietta, Oklahoma. The airplane, flown by an ATP rated pilot, was landing after a 14 CFR Part 91 personal flight from Oklahoma City. There was no flight plan filed and visual meteorological conditions prevailed. The pilot and one passenger received minor injuries while the two remaining passengers were not injured. According to the pilot, he was taking his family to dinner at a restaurant located adjacent to the private strip. Upon arrival in the area, he inadvertently overflew the airstrip and his son pointed it out to him. He subsequently turned around and flew parallel up the east side of runway 35. He then turned westbound for about five miles and executed a 180 degree turn. He entered left traffic for runway 35 and executed what he said was a normal downwind, base and final. He stated that he checked the runway for obstructions, both during the pass up the runway and during the downwind leg. He noted two aircraft parked on the west side of the south end of the runway and saw no other obstructions. The pilot further stated that the final approach and touchdown were normal and that when he lowered the nose, he saw there were cattle on the runway. He attempted to abort the landing and swerved to avoid the animals. However, as he raised the nose gear back off the runway, the airplane nose and nose gear struck a large calf. The calf hung up in the nose landing gear and the pilot decided to stay on the ground. During the landing roll, the airplane veered off the left side of the runway due to the damage to the nose gear. The pilot's efforts to maintain directional control with rudder and asymmetrical power were not successful. Investigation revealed that a road crossed runway 35 about one third of the way up from the south end. The road had cattle guards on both sides of the runway, however, the guards had become filled with rocks and dirt, allowing the cattle to cross. The airport owner, who owned the cattle, stated that he was not aware that they could get on the runway.