USA: Rachel Alexandra, one of the most popular US racehorses of the modern era, has been retired after failing to recapture the form of her stellar horse of the year campaign as a three-year-old.

Majority owner Jess Jackson released a statement on Tuesday night revealing the shock news that ‘America's Sweetheart', who had been expected to be aimed at the Breeders' Cup Ladies Classic, will instead go to the paddocks.

The four-year-old daughter of Medaglia D'Oro will be bred to Jackson's two-time horse of the year Curlin at his Stonestreet Farm in Lexington, Kentucky.

After a brilliant workout on Monday morning for trainer Steve Asmussen, Rachel Alexandra was thought to be on target for Saturday's Grade 1 Beldame Stakes at Belmont Park until Jackson dropped his bombshell.

"As you know, despite top training and a patient campaign, Rachel Alexandra did not return to her 2009 form," said the owner.

"Ibelieve it's time to retire our champion and reward her with a less stressful life.  We are delighted that she will retire healthy and happy to our beautiful farm in Kentucky."

Originally trained by Hal Wiggins, who owned her in partnership with breeder Dolphus Morrison and partner Mike Lauffer, Rachel Alexandra was purchased privately by Jackson and Harold McCormick after her runaway victory in last year's Kentucky Oaks.

She retires with a career record of 13 wins at eight different tracks plus five seconds from 19 starts, earning just over $3.5 million in prize-money and setting a handful of stakes-record times and superlative winning margins.

However, after a celebrated 2009 campaign during which she went unbeaten through eight races, including several against her male counterparts, Rachel Alexandra has suffered defeat on three of her five starts this term.

"Rachel Alexandra owes us nothing," added Jackson. "As a three-year-old, she set standards and records that no filly before her ever achieved.

"And I suspect it will be quite a while before a three-year-old filly ever equals or surpasses her achievements.

"Although her fans were thrilled by a series of spectacular victories, I believe they, as we, were simply awed time and again by her sheer beauty, courage and athleticism.

"Now we are looking to the future. We are eager to introduce Rachel to Curlin. Imagine what possibilities those two super horses might produce."

Put simply, Rachel Alexandra's 2009 campaign was one of the greatest ever seen from a female horse in the history of US racing.

Ridden solely last season by Calvin Borel - "the great Calvin Borel", according to Jackson - she not only beat the fillies by 20 lengths in the Kentucky Oaks and 19 lengths in the Mother Goose Stakes, but also beat the colts with famous victories in the Preakness, the Haskell Invitational and the Woodward Stakes.

In the process, she became a household name in the States and beyond, known simply as ‘Rachel' to her legion of fans, who instantly recognised her by her distinctive upside-down exclamation mark white blaze on her nose.

Off the track, she helped raise money for cancer research, education and other charitable causes, and was the subject of a photo spread in the August 2009 issue of Vogue magazine. She also has her own website and Facebook page.

Although they were never to meet, a fierce rivalry developed between this great filly and her west-coast counterpart, Zenyatta, who also compiled a remarkable unbeaten record that stands until this day.

The prospect of a clash between this formidable duo was the talk of American racing circles for much of last year, but instead Rachel Alexandra's season ended after her narrow victory in the Woodward, leaving Zenyatta to beat the colts in the Breeders' Cup Classic.

In a somewhat controversial result, Rachel Alexandra was voted horse of the year in front of Zenyatta at the Eclipse Awards ceremony for 2009.

A projected meeting between the pair in a $5 million showdown for the Apple Blossom Stakes at Oaklawn Park in April came to nought after Rachel Alexandra suffered a shock defeat at odds of 1-20 on her seasonal debut at the Fair Grounds in New Orleans.

She was then beaten again, in a Grade 2 event at Churchill Downs on Kentucky Oaks day, before finally regaining the winning thread by ten lengths in another Grade 2 contest, the Fleur de Lis, and an ungraded stakes race at Monmouth Park in July.

She was beaten again, however, when facing off in Grade 1 company for the first time this term in the Personal Ensign Stakes at Saratoga in August.

She performed creditably in defeat, duelling through hot early fractions before just being run down close home and beaten a length by the outsider Persistently over a 1m2f inexcess of her optimum.

Such an admirable performance gave rise to legitimate hopes among her fans that she would finally score an overdue Breeders' Cup win on the dirt at Churchill Downs in November.

Now, though, the Personal Ensign turned out to be the final race of a glittering career.

Trainer Steve Asmussen paid tribute to Rachel Alexandra. "I have been blessed to have been part of history," he said.

"We areall very fortunate that Rachel carried the banner following Curlin's amazing success story. The fans adored her - we all did. She had the most fluid and beautiful stride of any horse I have every seen. It's been quite a ride."