Even though the Kentucky Derby is still run on a dirt track, synthetic racing surfaces such as Polytrack are a factor in this year's Run for the Roses.Nearly half of this year's Derby starters ran their last race on a synthetic surface and two of them, including highly regarded Colonel John, are making their dirt track debut.Handicappers say the new tracks are making it tough to evaluate the thoroughbreds.Some have performed well on Polytrack and similar surfaces, while it's blamed for poor performances such as that put in by Pyro in Keeneland's Bluegrass Stakes.NBC Sports racing analyst Bob Neumeier says the reviews for synthetic tracks are mixed."I think it is a bit of a contoversy within the industry in a sense that the trainers and owners and some track management people like it. They say its safer for the horses and that may all be true. but the players haven't figured it out yet," Neumeier said.Most North American racetracks still use dirt, but many of the Derby prep races were run on synthetic tracks.