Sir Shackleton, a homebred millionaire for Tracy Farmer, has been retired from racing because of an injured sesamoid. The 5-year-old son of Miswaki was a stakes winner all three years he raced."He had a good career, but it came time to retire him," said Farmer's adviser, Lincoln Collins.Named for Antarctic explorer Sir Ernest Shacketon, Sir Shackleton ran the fastest seven furlongs at Gulfstream Park in 2005, a 1:21.64 clocking, in winning the Richter Scale Breeders' Cup Sprint Championship Handicap (gr. II). Later that year in New York, he ran a solid second to subsequent Horse of the Year Saint Liam in the Woodward Stakes (gr. I) and also placed in the Metropolitan (gr. I) and Whitney (gr. I) Handicaps.As a 3-year-old in 2004, Sir Shackleton won the mile Derby Trial Stakes (gr. III) at Churchill Downs and the 1 1/8-mile West Virginia Derby (gr. III). In his debut this year, he set a track record of 1:22.28 in winning the Super Stakes at Tampa Bay Downs.Sir Shackleton, who was produced from the Star de Naskra mare Naskra Colors, retired with seven wins from 20 starts and earnings of $1,051,500. Breeding plans will be announced later.