Before racing at Leopardstown yesterday, Aidan O'Brien cast definite doubts over the Derby winter favourite, Kingsbarns, making the line-up for the Epsom showpiece. Once the sport started another of the Ballydoyle contenders for the premier Classic, The United States, did the same on the track for his own blue-riband prospects. It was left to Battle Of Marengo to pick up the standard for the powerful Co Tipperary stable, a task he completed in some style in the Ballysax Stakes.

The 10-furlong contest has a fine record as a second-season starting point for horses O'Brien holds in high regard; his previous six winners include Galileo, High Chaparral, Yeats and Fame And Glory. Whether Battle Of Marengo can emulate the first two named in the Derby remains to be seen, but his performance yesterday provided little to cavil about as he coped with a 5lb concession to his four rivals, rain-softened going and a strong headwind.

Such conditions would not be conducive to instant, brilliant acceleration but after being sent to the front early in the straight by his trainer's son, Joseph, the 8-11 favourite went up through the gears with an efficiently determined echo of his name and at the line had a length and three-quarters to spare over Sugar Boy, winner of his three previous starts.

"Very happy with that," O'Brien said. "The ground was not ideal at all but we needed to get going with him. He's always been one of our nicest, a lovely sort of horse who has passed pretty much every test. He's a beautiful-moving horse with a big, open stride and he'll be better on better ground."

Battle Of Marengo, likely to return to Leopardstown next month for Ireland's next traditional Derby trial, the Derrinstown, is now as short as 8-1 for the Epsom Classic. Two of his stable's other sons of Galileo are also prominent at the sharp end of the market: Mars, a possible for the 2,000 Guineas, and, despite his trainer's warnings, Kingsbarns.

The winner of last year's Racing Post Trophy, Kingsbarns had already been ruled out of the 2,000 Guineas after a training setback (a hoof became infected after he pulled a shoe off at exercise last month) but as recently as Friday, O'Brien was still bullish about getting him to Epsom on 1 June. Yesterday, however, he was much more guarded. "He was totally stopped in his work for 10 days," he said, "and when it happened was far from ideal. We are struggling a bit to get him 100 per cent and it's time we are now fighting with him."

If the colt makes it to a trial, it will be at the 11th hour. "We want to give him as much time as we can to get back," said O'Brien, who also won yesterday with Lockinge Stakes-bound four-year-old Declaration Of War and handicapper Justification. "He's in the pool a lot and we're forcing as much as we can without risking the horse. Of ours, he was the horse we were dreaming about but things just haven't gone well with him."

The United States had been the subject of some support at longer prices for the Derby, but his prospects effectively disappeared with a dull effort in the 2,000 Guineas trial. The winner, Fort Knox, considerably enhanced his Classic prospects but the main pointer it provided was to what a trainer his rider Johnny Murtagh will make in due course.

Fort Knox, a brother to 2011 Guineas runner-up Dubawi Gold, transferred as a maiden winner during the winter from Richard Hannon to Murtagh's operation, headed by Tommy Carmody, on the Currragh. Yesterday's victory, after a sweeping run from last to first in the straight, resulted in a cut to 20-1 from 50-1 for the Rowley Mile Classic next month.

Turf Account

CHRIS MCGRATH'S NAP: Destiny Blue (3.40 Redcar)

Failed to cut much ice as a hurdler last year but was useful on the Flat two years ago and, to judge by his reappearance in that sphere over today's course and distance, that ability remains ready to be tapped by his thoroughly capable new trainer.

NEXT BEST: Bitusa (3.10 Redcar)

Shaped with promise last year and on his reappearance two weeks ago and can offer more for return to seven furlongs on better ground.