Belmont Park yesterday opened the fall race meeting that has traditionally determined most of America's thoroughbred champions. But any traditionalist who looks at the stakes races on the schedule might find them unrecognizable and wonder whether this country is now producing fainthearted horses.

The Woodward Stakes has been shortened from 1 1/4 miles to 1 1/8 miles. The Beldame Stakes, the most prestigious event for fillies and mares, also has been shortened by a furlong. The famed Jockey Club Gold Cup has been pared from 1 1/2 to 1 1/4 miles.

The evolution of the Gold Cup epitomizes a trend in U.S. racing. From 1921 to 1975, it was contested at two miles, and a thoroughbred who wanted to earn the horse-of-the-year title was expected to prove his ability at that distance. It was not a race for plodders. Horses with great speed, such as Citation, Nashua and War Admiral, successfully carried that speed for two miles.

By 1976, however, two-mile races had become an anachronism, and the distance of the Gold Cup was reduced to 1 1/2 miles. Now, it seems, even that distance is too much for modern horses. With the shortening of the Gold Cup, the Belmont Stakes is the last remaining 1 1/2-mile dirt race of any real importance in the United States.

This change is occurring even though longer-distance races remain very popular with the public -- the Belmont Stakes annually draws the biggest crowds in New York. So why are these races disappearing? Are horses less robust than they used to be?

Leonard Hale, vice president of the New York Racing Association, said that the changes in Belmont's big fall races were made largely to make them serve as appropriate prep races for the Breeders' Cup. The $3 million Breeders' Cup Classic is at 1 1/4 miles and, Hale said, trainers don't want to run their horses at a longer distance in a prep race. But, he acknowledged, trainers don't seem to want to run at longer distances, period.

One explanation is that horses can make fewer starts if they are running in tough distance races. Just about everybody who runs in the Belmont, for example, gets a lengthy breather thereafter. That means they may have to sacrifice money-making opportunities.

"We have so much racing now that every weekend you can put a horse on a plane" for a rich race, Hale said.

John Veitch, who trained Alydar for the greatest 1 1/2-mile race of modern times, the 1978 Belmont, agrees that members of his profession are not developing distance runners.

"I think trainers are lacking -- not the horses," Veitch said. "If a horse can go a mile and a quarter, he can go a mile and a half; it's just a matter of training. But most guys don't have a concept of how to get horses ready to go a mile and one-half. It's a type of skill that's not called on much any more."

When a trainer does prepare a horse properly for a longer-distance race, he will develop the animal's stamina at the expense of quickness -- which can be a drawback. Easy Goer won last year's Gold Cup impressively, but when he went into the Breeders' Cup Classic, he seemed to have lost his zip, and he was outkicked by Sunday Silence at the critical stage of the race. Plenty of smart racing people blame the loss on the fact that he was coming out of a 1 1/2-mile race.

If most trainers are not training horses to be distance runners any more, breeders are not breeding them to go longer distances either. Plodders have never been considered desirable stallions. Just about everybody subscribes to the philosophy of the late Aga Khan, a world-famous breeder, who was once asked the three most important traits in a sire, and replied, "Speed, speed, speed."

Of course, there are plenty of stallions who combine brilliant speed with distance-running ability (e.g., Seattle Slew), but quick horses with questionable stamina always get much more of a chance at stud than those with opposite qualities. And stallions may be in great demand even when their shortcomings in stamina are well known. Mr. Prospector was strictly a sprinter, and his offspring tend to be milers, but he nevertheless has been North America's leading sire two of the last three years.

John Nerud, a knowledgeable breeder and a Hall of Fame trainer, thinks U.S. breeders are producing inferior horses nowadays, and places the blame on basic changes in the industry.

Years ago, the most important breeders raced their own horses. They planned matings to create the proper blend of speed and stamina. But now that commercial breeders dominate the game, and most well-bred horses wind up at yearling sales, Nerud said: "Everyone is breeding pedigrees that will sell. They're not breeding with any specific ideas in mind."

However, the types of racehorses that breeders produce are determined by the money-making opportunities that those horses will have.

Bill Oppenheim, editor of the breeding-industry newsletter Racing Update, said: "If we regard a mile and one-quarter as the outside limit of our races, then we're not going to have horses who can go a mile and one-half. It becomes a self-fulfilling prophecy."

Oppenheim noted that even in England, where races at two miles or more have long been commonplace, there is a similar trend toward shortening races. "At the rate we're going," he said, "we may see the English Derby {now 1 1/2 miles} at a mile and one-quarter and the Kentucky Derby at a mile and one-eighth."

Unthinkable? There was a time a shortened version of the Gold Cup would have been unimaginable too.