The names of Darley, Shadwell and Juddmonte feature on a list of 40 owners who have pledged not to use raceday medication, including Lasix, on their two-year-olds in 2012.

In a highly significant move, a number of major owners have vowed not to use the anti-bleeding medication furosemide (more usually known by its former trade name Lasix) and adjunct medication on juveniles.

The move, announced in a statement issued by the US Thoroughbred Owners and Breeders' Association (TOBA) on Thursday, is seen as a first step in moving away from the pervasive laissez faire attitude towards medication on American racetracks.

It follows the decision of the Breeders' Cup to ban such medication in two-year-olds races at Santa Anita in November.

The use of Lasix remains a controversial subject in American racing, where the vast majority of horses race on the medication, which is banned in other major racing nations.

However, several US trainers have spoken out against any ban, so such a message of support from a number of large-scale owners is a hugely significant development.

Among those to support the move are Team Valor, George Strawbridge, William S Farish, Robert Clay and Bill Casner.

"The use of race-day medications has grown to the point where nearly every horse in every race is being treated just hours before they go to the saddling paddock," said TOBA chairman Peter S. Willmott.

"This practice is not in the long-term best interests of the horse, nor is it the proper message we need to communicate with our fans if we wish to increase the popularity of the sport.

"We should all take steps to reverse the use of raceday medication and at this time the best way is to encourage like-minded owners to race their two-year-olds medication-free."