The pilot, who was located in the front seat of the tail wheel equipped airplane, had just completed two uneventful full stop landings at the private grass airstrip. The pilot in the rear seat then took the controls, and departed with the intention of performing a full stop landing. The rear seated pilot stated that during the landing, cattle ran from behind a line of trees, and onto the runway. He did not have enough separation from the cattle to perform a go-around, so he aggressively applied the brakes. The airplane subsequently nosed over causing substantial damage to the wings.