The Washington International Horse Show, which will open today at 5 p.m. at Capital Centre, has gone country.

Country singers Johnny Cash, Lynn Anderson and June Carter have been enlisted to perform in an effort to attract larger crowds. They'll handle the singing; riders such as U.S. Olympic equestrian gold medalist Joe Fargis and West German rider Klaus Reinacher will handle the jumps.

"We've done a number of new things this year," said second-term show president Julian Heron. "We figure if people come out to see Cash and Carter, and they haven't seen the jumping class, they might like it. We want to attract new people."

The international jumping competition, which always has been the backbone of the show, will once again take place at each session. Representatives from West Germany, Great Britain and Canada will go up against the United States Equestrian Team. This year's U.S. team includes defending President's Cup champion Fargis on Touch of Class, Louis Jacobs, Katie Monahan and Michael Matz.

The eight days of competition culminate next Sunday with the $40,000 Michelob President's Cup.

In the hunter classes, where the horses are judged (not scored) on their performance and, occasionally, their conformation, last year's grand champion, Just For Fun, will return to defend the title. Just For Fun again will be ridden by Charlie Weaver of Cismont Manor Farm in Keswick, Va.

In the classes for riders under 18 years of age, Mary Lisa Nicholson of Gaithersburg, winner of the best-child-rider award last year, is favored to repeat. Nicholson is scheduled to ride the champion junior hunter, Upon A Star, in addition to a number of other mounts.

Nicholson has been nicknamed "The Robot" by fellow riders on the circuit. "My friends call me that because of my ability to consistently have one good round after another on horses I've never ridden before," Nicholson said.