Maryland's fall thoroughbreds racing season starts tomorrow at Bowie, facing extended competition from the trotters for the first time but with the three major tracks apparently agreed on 1979 racing dates.

Bowie's 36-day meet, using the dates it purchased from now defunct Mariboro Race Course, will run through Oct. 20. Laurel Race Course opens the next day and closes Dec. 30.

Horsemen will be watching these two meetings closely - for the impact of harness racing (through Nov. 20) and for the impact of higher purses (more than $20,000 daily) from the increased pari-mutuel takeout that went into effect July 1.

The state legislature also banned the 48-day summer meeting that the tracks had been pooling and running at Bowie and Pimlico every other year. As a result, the 1979 meetings at each track probably will last 76 days each (up from 60 this year), plus the Timonium and Marlboro-at-Bowie dates.

As of late last week, the 1079 dates appeared to be set in the following order, according to racing industry sources: Bowie 76 days, Pimlico 76 days, Timonium 45 days, Marlboro-at-Bowie 36 days and Laurel 76 days.

Harness racing in Maryland has come into its own in the past two years, first with the introdution of the Harness Racing Board as the equal of the Maryland Racing Commission's Thoroughbred Racing Board, then with the nifty efforts in Annapolis that got the troters 54 additional days this year.

The thoroughbred people are apprehensive, with Laurel Race Course the most outspoken and the Bowie people merely saying that the trottes siphon off some gambling dollars as bingo would.

Frank Brady, Laurel Race Course general manager, hits the real problem head on:

"Any track running at night would handle more money than a daytime track, conditions being equal."

Until recently, the harness tracks were thought of as nothing more than a petty nuisance, surely not a major competitor. But now the trotters have representation and the prime-time hours, if not yet public acceptance and big-league horses.

Laurel Race Course President John Schapiro made overtures recently about switching to night racing, which Brady estimates would cost the track about $2.5 million for air-conditioning and lighting.

Joe Shamy, who runs Laurd Race-way with his wife, Greta, minces no words when asked what would happen if one of the thoroughbred tracks switched to night racing.

"There will be war," he said. "I'll tell you that. Two commissions (thoroughbred and harness) will go at each other's throats. It's an unwritten law. If Laurel Race Course goes nights, we'd like to go Saturday afternoon, too. That's the biggest racing day of the week."

The Laurel Raceway meeting, which floundered during the vacation month of August, runs through Oct. 7. Then Rosecroft opens Oct. 10 for 36 nights, 18 of its own and 18 belonging to Ocean Downs, the state's other harness-racing track.

That is the meeting on which all eyes will be glued. Late spring usually has been the best time of the year for harness racing Racing and entertainment experts expect a windfall in this previously untapped source, as long as Rosecroft can get enough quality horses and avoid the cheaper ship-ins that hurt Laurel in August, when Liberty Bell opened.

As a result, the harness people have not even begun talks about 1979 racing dates. Each track will have 85 dates and Ben Schwartz, chairman of the Harness Racing Board, wants to avoid the two Laurels running head-to-head, as would be the case if Rosecroft opened the season with its 85 nights followed by Laurel.

Shamy says he and his wife prefer one meeting because it is "too expensive" to close and reopen his plant for a split meeting. But, he added, another 15 nights of racing, increasing each track's dates to 100 annually, would adapt well to split meetings.

Under that format Maryland would have 300 harness-racing dates, only nine fewer than thoroughbred racing's 309 (the thoroughbred tracks have only four dark days, other than Sundays, a year).

However, Schwartz said he is vehemently opposed to additional dates.

"Eighty-five is plenty at any facility," he said. "There's such a thing as getting to be a pig. As long as I'm chairman and can convince my board I'm right, I'll never ask for more than 85 dates."

Schwartz also said Shamy's "war" cries are ill advised at this time.

"He's looking for problems and there's no problem there. At this point, that just isn't the case. But, if the thoroughbred board approved night racing, which I don't think they would, we would go with every weapon we have, since we are in the minority.

"But this was not the intent of creating the harness board. Our function is to keep peace. We'd be defeating each other. If we go fighting among each other, then we will be in the same boat as Liberty Bell and Brandywine (two harness tracks in the Philadelphia area). They're going head on and killing each other. Does this make sense?"