A clash of schedules has forced Zac Purton to abandon former dual horse of the year Ambitious Dragon for incumbent Military Attack and left trainer Tony Millard with a headache ahead of the season's majors.

The two regular Purton mounts are due to clash next month in the Group Two Sha Tin Trophy (1,600 metres) after which their paths will likely diverge as Military Attack moves to a middle distance diet and Ambitious Dragon remains at the mile.

I've decided to go with Military Attack. Hopefully, at some stage in the season, the connections of Ambitious Dragon might decide they would like me to ride their horse

However, connections of both horses have insisted on a full commitment from Purton, who agonised over the choice between the best two active horses in Hong Kong before deciding on the one more likely to light up his name internationally.

Purton won the S$3 million (HK$18.35 million) Singapore International Airlines Cup in May on Military Attack in a hand canter, and the campaign sketched out by trainer John Moore for the five-year-old includes a defence of that title and a World Cup tilt along the way.

Moore has a clear, and potentially lucrative, path mapped out for Military Attack, chasing the sharp end of more than HK$150 million in stake money - in the Jockey Club Cup second-up, followed by the Longines Hong Kong Cup, Hong Kong Gold Cup, Dubai World Cup, Audemars Piguet QE II Cup and a return to Singapore.

"With the bigger picture in mind, I've decided to go with Military Attack. Hopefully, at some stage in the season, the connections of Ambitious Dragon might decide they would like me to ride their horse, but that's a decision for them to make," Purton said yesterday.

"I thank Tony Millard and the Lam family very much for giving me the opportunity to ride Ambitious Dragon. We had a fantastic season winning two Group Ones and I will always be grateful to them. Unfortunately, I can only ride one horse in a race."

Moore, for whom the jockey is also likely to ride top sprinter Frederick Engels this season at the request of owner Martin Siu Kim-sun, said Military Attack's owner, Steven Lo Kit-sing, had asked that Purton make up his mind.

"I did put Zac on the spot, otherwise we were going to make plans to bring Tommy Berry in to ride the horse in the Group One races," Moore said. "Military Attack and Ambitious Dragon possibly won't meet again after this first race but we wanted Zac all the way through."

Millard said he had received calls from jockeys keen to ride the reigning Hong Kong Mile champion if Purton decided to go with Military Attack, but added no firm decision had been made on a replacement.

"I didn't want us getting caught without a jockey and without options further down the line, so I wanted Zac to make a choice," Millard said.

While the Jockey Club allows owners to fly jockeys in to ride in Group One races, and the designated lead-up with a commitment to ride in the main event, Millard's options for the Sha Tin Trophy under handicap conditions would be almost limited to the riders already here.

Two of them - Douglas Whyte and Weichong Marwing - have previously been sacked from Ambitious Dragon. There is also the question of prior commitments at a time when jockeys and apprentices number just the bare 20 until the arrival of Gerald Mosse, Brett Prebble and Neil Callan over the next two months.