The Laurel stewards have scheduled a meeting Thursday with apprentice jockey Mark Johnston as a result of yesterday's ninth race in which heavily favored One Third was barely beaten at the wire after Johnston failed to ride One Third aggressively through most of the stretch.

One Third, the 11-to-10 favorite, had about a three-length lead in midstretch under a good hold by Johnston. From the top of the stretch, Johnston three times glanced back at his closest pursuers, McKilts and St. Haven, and maintained his hold. He did not use his whip.

Within the final sixteenth, McKilts rallied under Joe Rocco. As she angled off the rail and advanced outside of One Third, Johnston -- about five strides before the wire -- began driving his filly hard, but it was too late. McKilts beat her by a nose and paid $16.60.

Johnston, at age 19 the nation's leading rider with 239 victories, appeared badly shaken afterward. He indicated he thought he had the race won and therefore sought to save his filly.

"I messed up," he said. "She {One Third} was running well under wraps. I looked back and saw Rocco and Edgar {Prado, astride St. Haven}. I could see Edgar's horse wasn't running."

Johnston, who has ridden for about 14 months, was booed vigorously as he brought Lovely Bones onto the track for the final race. Laurel's top jockey with 36 victories over the 26-day meeting, Johnston has one week remaining in his apprenticeship.

The stewards declined to comment until they have reviewed films of the race with Johnston Thursday morning.

The difference in purses between first and second place was $7,220.

Hurting at Charles Town

The past week has been hard on several Charles Town jockeys. On Monday, Orlando Castaneda became the third rider injured in a six-day span, suffering a broken left femur when he was thrown following a workout. Castaneda, 54, was scheduled to have a steel plate inserted in the injured thigh yesterday.

Leading rider Tim Marchant suffered a broken right ankle in a car accident last Wednesday and Lori Bourne sustained a broken back and pelvis when a horse flipped on her at Charles Town Sunday.

Bourne, who has ridden occasionally in Maryland the past few years, is expected to remain at Washington Hospital Center for at least three weeks. She reportedly suffered no spinal damage.

Bourne, 23, had been having her best year ever, finishing sixth during Charles Town's spring meeting with 33 victories. . . .

Ten Keys became the seventh Maryland-bred to surpass $1 million in earnings with Sunday's victory in the New Hampshire Sweepstakes, setting Rockingham Park's 1 1/8-mile turf course record of 1:47 3/5.

Now 20 for 50, Ten Keys has become the most valuable claim in Maryland racing history, trainer Mike Pino having taken him for $14,500 in early 1987. With his fourth win in six tries this year, Ten Keys moved into fourth place among Maryland-bred earners at $1,125,211. He trails Broad Brush, the faraway leader at $2,656,793, Little Bold John ($1,852,266) and Homebuilder ($1,172,153). He is about $10,000 ahead of Safely Kept.

Grand Canyon Put Down

Grand Canyon, one of the top juveniles of 1989, was destroyed after contracting laminitis, the Daily Racing Form reported. A son of Fappiano, Grand Canyon won the Hollywood Futurity last fall in 1:33, the fastest mile ever run by a 2-year-old in North America. He had not raced this year. . . .

The Maryland Horse Breeders Association announced its support for the use of therapeutic medications, but called for further research into the inheritance of certain physical conditions among racehorses. The MHBA stated its position in response to a recent University of Pennsylvania study that in part determined Lasix a performance-enhancing medication that is often ineffective in preventing respiratory bleeding. . . .

Jockey Eric Saint-Martin, son of French riding champion Yves Saint-Martin, is coming to Maryland. Agent Derek Lawson said the 24-year-old journeyman should arrive from France today and may begin riding this weekend.