LEXINGTON, Ky. -- Conquistador Cielo, 1982's Horse of the Year, was euthanized Tuesday morning at Claiborne Farm, where he stood throughout his long stud career. Conquistador Cielo was 23.

The Mr. Prospector stallion underwent colic surgery earlier this year and recovered well from that but injured his right knee during his recuperation. He subsequently developed founder in his left front leg, a common stress-related hoof condition in horses that shift their weight from an injured leg onto a healthy leg. Age-related arthritis in one hip compounded the stallion's difficulties.

"He foundered, and the founder became acute on Monday," Claiborne manager Gus Koch said Tuesday. "At that point we knew that we would not be able to control his pain, and we did the best thing for the horse. We were fortunate to have Conquistador Cielo for his entire career and are proud of his accomplishments."

During his championship season in 1982, Conquistador Cielo won the Grade 1 Belmont Stakes and Metropolitan Handicap, the Grade 2 Dwyer Stakes, and the Grade 3 Jim Dandy Stakes. He also finished third behind longshot Runaway Groom in the Grade 1 Travers, which was his final race. That campaign netted Conquistador Cielo top 3-year-old honors and the Horse of the Year title. Trained by Woody Stephens for Henryk de Kwiatkowski, Conquistador Cielo ran for two seasons, completing his career in 1982 with a record of 13-9-0-2 and $474,328 in earnings.

Claiborne and de Kwiatkowski syndicated Conquistador Cielo for a record $910,000 a share, for a total valuation of $36.4 million. De Kwiatkowski retained 10 shares. Florida-bred Conquistador Cielo sired such runners as multiple Grade 1 winner and $2 million earner Marquetry; Grade 1 winners Wagon Limit, Norquestor, and Conquistarose; millionaire Forty Niner Days; and graded stakes winners Alannan, Mi Cielo, Lexicon, Unrullah Bull, and Truluck.

At the time of his death, Conquistador Cielo had 17 crops to race for total progeny earnings of $52,649,774.

Conquistador Cielo, a son of the Grade 3-placed Bold Commander mare K D Princess, was buried Tuesday at Claiborne's Marchmont Cemetery. His grave is next to Unbridled's.