(defendant’s own farm ground on which they keep horses and plaintiff hired to perform maintenance work and care for horses at horse center in Lexington; defendant asked plaintiff to come to defendant’s farm to assist in trail ride on defendant’s property; plaintiff injured on trail ride and sued under premises liability theory – negligence for failure to mow tall grass and/or remove debris; trial court granted defendant’s motion for summary judgment; appellate court affirmed on basis that injury caused by natural outdoor hazard that was apparent and obvious to plaintiff; defendant did not breach duty of care owed to an invitee).