The show ring at Broadway, Va., is full of dust and horses, and the stands are full of loud and sweaty partisans

"There's your winner, Judge," booms a powerful male voice in support of a coal-black Tennessee Walker.

"Lookie here, you see that little roan as good as me," retorts a woman whose voice is gravelly as a country road.

Such shouting matches are typical of the "Plantation Pleasure" class, a horse show event too new to have become as elaborately couth as the ones in which the debutante daughters of the horsey set complete. The spectators are more likely to bring their picnic lunches in old cardboard boxes from the Safeway than in wicker baskets from Camalier & Buckley, and they eat fried chicken and biscuits rather than pate and Brie.

"These gaited-horse shows bring out Mother's side of the family in me," said the roan's partisan. "Mother's people were from outside Baltimore . . . They were, ah, more county fair than the country club."

The black horse's champion, a seasoned trainer, dismisses the loud minority who are cheering the roan as "newcomers." He invites all once again to admire the black, which can't seem to stop bobbing its long, serious head in the deep nod that is part of his unique gait. The veteran showhorse "makes fewer screwups than John Foster Dulles," the trainer says.

The show, sponsored by the Broadway Lions Club, is one of the remnants of a Deep South circuit popular in the '30s and '40s. Also featured are the American Saddlebred - the 19th-century equivalent of the sports car - prancing roadster ponies and high-stepping Tennessee Walkers.

The Plantation Pleasure class is a gentler event for the Tennessee Walkers that grew out of the "soring" scandal of a decade ago, in which trainers applied painful chemicals to the horses' legs to accentuate their floating gait. Plantation horses are judged mainly for comfort and versatility. And at the intermediate level there are Park Pleasure classes for slightly more stylish walkers. Park classes also grew out of the reform movement.

Truman Bolkart of Eugene, Mo., trainer of the gelding Overlord, 1977 Plantation Pleasure world champion, says thereform movement has created a brand-new market for breeders. "The old-type show horses are expensive to keep," he said. "They need special shoeing, year-round care and professional training. Plantation horses are a poor man's show horse. Anybody can afford one, and nowadays he can show 'em, too. They are the coming thing." He says this with a trace of regret, since the new classes are cutting into his specialty of training classic Tennessee Walkers.

Peggy Snoddy, who manages entries for "The World's Largest Horse Show - The National Walking Horse Celebration," said her office now gets more requests for the pleasure horse classes than the traditional "show horse" events. "The pleasure horse folks want to feel like the Celebration is as much theirs as the show-horse people's. This year they want us to add western-saddle events."

At their most refined, the gaited-horse people are local gentry showing horses they have raised and trained themselves. At their loudest they are very like Republicans in the New South, exuberant over finally being taken seriously. Their riding coats are bright brocades rather than the austere black meltons worn in the hunter classes. And it seems to be mostly first- or second-generation money they're throwing around; the horse people who have forgotten where the family fortune came from seem to be sticking with the Thoroughbreds.

The pleasure-class riders are mostly men, and mostly older than the mannered youngsters who ride Thoroughbreds. But today, at Broadway, there is an exception: The winner is the flashy roan, ridden by a middle-aged woman with a beehive hairdo who coolly guides her mount to the trophy table. A gust of wind disarranges the elaborate coiffure, but still she hangs tough, as she has all afternoon in this tough class.

Twenty more riders follow her out of the gate; one 16-year-old rider waves her green sixth-place ribbon at a group of friends.

"Look!" she shouts sarcastically, "I won two dollars." The even less successful competitors continue to flow by, ribbonless.

The girl reflects for a moment and then smiles. "Well, it beats nothing."