Leading Horse of the Year contender Mineshaft was retired Friday because of a minor ankle injury and won't run in the Breeders' Cup Classic.

Trainer Neil Howard said racing's newest star has a chipped bone in his right front ankle, an ailing right knee and a bone chip developing in his left ankle.

"We really had no alternative but to retire the horse at this time,'' Howard said Friday, just hours after watching Mineshaft take a morning gallop at Churchill Downs. "The problem is such that you'd never know anything was wrong until he was under the stress of a race.''

Mineshaft, owned by William S. Farish, will stand at Farish's Lane's End Farm near Versailles, Ky., and will command a $100,000 stud fee.

Rather than risk serious injury, the decision was made to end Mineshaft's career one race early -- the colt was scheduled to be retired after the BC Classic.

With a command performance in winning the Jockey Club Gold Cup at Belmont last week, Mineshaft became the front-runner for Horse of the Year honors. Even without running in the BC Classic at Santa Anita on Oct. 25, the colt remains the top contender over the likes of Medaglia d'Oro and Perfect Drift.

Mineshaft has won seven of nine races with two runner-up finishes this year for earnings of more than $2 million. The 4-year-old son of 1992 Horse of the Year A.P. Indy ends his career with 10 wins in 18 starts and earnings of $3,454,720.

Mineshaft came into the Gold Cup off wins in the Woodward Stakes and Suburban Handicap, the race in which Howard said the colt first developed the chip in his right ankle.

"We've monitored it very closely after the Suburban and thought it was manageable,'' Howard said. "But when we got him home after the last race, X-rays revealed that the chip had broken off and there was degeneration in the right knee and the other ankle was developing a chip.''

Belmont Stakes winner Empire Maker was retired earlier in the week.