Irish trainer Aidan O'Brien believes Declaration Of War offers something different that might just make a difference in the Breeders' Cup Classic.

O'Brien has tasted plenty of success in the Breeders' Cup, but victory in the $US5 million ($A5.30 million) marquee race has eluded him - so much so that O'Brien said on Thursday he no longer allows himself to imagine winning it.

"We know how important this race is and we've tried with so many horses. But at this stage I don't even dream about winning the Classic," he said.

The trainer has seen such superlative horses as Galileo and Giant's Causeway go down in defeat and witnessed George Washington being fatally injured at muddy Monmouth Park in 2007.

It's a tale of futility that US trainer Bob Baffert can certainly identify with. Baffert has trained eight Breeders' Cup winners, but never a Classic champion.

This year, Baffert hopes the third time will be the charm for Game on Dude, last year's well-beaten favorite and runner-up the year before in the 1-1/4 mile (2000m) race that caps the 14-race, $27 million Breeders' Cup carnival at Santa Anita Park on Friday and Saturday.

Baffert believes maturity will make a difference for Game On Dude, who is a "much different horse" this year.

"I think mentally he's settled down a lot," Baffert said of the early 8-5 favourite.

"He's really mellowed out. "I think he's a much more relaxed horse. He's healthy. He's carrying more weight than he usually does. His races haven't been really, really hard on him."

Baffert has spaced out Game on Dude's races more this year in order to keep him fresh for the Classic and he comes into the race undefeated since finishing seventh in the race last year.

O'Brien, too, says a light racing schedule has Declaration Of War fresh for the Breeders' Cup, the result of a decision not to run him over heavy ground in Europe.

In fact, O'Brien said, it's been hard to find enough work for Declaration Of War, who hasn't raced since winning the International Stakes at York in August.

The unknown factor is how well Declaration Of War will handle the US-style dirt track but O'Brien is optimistic that solid dirt runners in his pedigree are an indication that he will.

"There's loads of speed on his outside, so he's going to be in there. It will be how he handles the kickback," O'Brien said.

The trainer took Declaration Of War to exercise with two other Group One winners on the synthetic track at Southwell, which he thought offered the closest approximation to a dirt track in England.

The Classic field also includes last year's winner Fort Larned and runner-up Mucho Macho Man.