The Melbourne Cup is off the agenda for New Zealand mare Silent Achiever.

The six-year-old multiple Group One winner turned in her best run of the spring, and one her trainer Roger James rates as one of the best of her career, when a close third in Saturday's $3 million Cox Plate.

Silent Achiever would have to carry 56.5kg in the Melbourne Cup, a weight that concerns James and has dampened his enthusiasm about heading towards the famous race.

He said he was also concerned about the possibility of a firm track.

Makybe Diva, in her third success in 2005, is only mare to have carried more than 56.5kg and win the Melbourne Cup.

The weight also puts Silent Achiever just 2kg away from Caulfield Cup winner and topweight, Japanese contender Admire Rakti.

"I think we can do better for her than ask that question of her," James said.

The decision to miss the Melbourne Cup was made after a discussion with the mare's owner-breeder Kevin Hickman.

"I'm going to sit down with him tonight and plan out the rest of her career," James said on Sunday.

Silent Achiever won the Group One Ranvet Stakes and BMW and was fourth in the $4 million Queen Elizabeth Stakes in Sydney during this year's autumn carnival.

Those races appear obvious targets again and James is also mulling over an international campaign.

The trainer was proud of Silent Achiever's performance to be beaten less than half a length by Adelaide and Fawkner on Saturday.

"She tried her heart out," James said.