Rockingham Park plans to close in a few years in what would mark the end of the state's first and only horse racing track. The owners of the 96-year-old track want to develop the 170-acre property in Salem for retail, office and residential use. ``It's the beginning of a new era in this location,'' Edward Callahan, the park's general manager, said Wednesday. He said racing at the track will continue for at least two years while the owners come up with a development plan. Betting at Rockingham fell by half after the Foxwoods casino opened in Connecticut 10 years ago, he said. The park has lobbied the state Legislature for years to legalize video slot machines at the state's horse- and dog-racing tracks. The track employs about 300 people in the offseason, when it has simulcast racing. It employs 500 to 600 people during the live racing season.