The sport, organised by the Pony Club involves riding, shooting, running and swimming.
Tracey, 19, of Highbridge, a sports coaching student at Hartpury College, near Gloucester, only learned that she would be competing six weeks ago, after another team member had to drop out through illness.
But she is raring to go after raising the money to fund her place, partly through her part-time job as a lifeguard at Burnham-on-Sea swimming pool.
Tracey, who is a member of Weston Harriers Pony Club, will, like other competitors, have to ride an "unknown" horse in America for the riding section of the competition. A former pupil of Taunton School, she represented Somerset at swimming as a 16-year-old.
Hayley Aston, a committee member of Weston Pony Club said: "Tracey was short-listed after taking part in a training camp at Easter and now she has the chance to go.
"This is a great achievement which should be recognised locally."