By The Associated PressLouisiana Downs in Bossier City, La., will turn on its slot machines May 21 well in advance of its live Thoroughbred meet that begins June 27.Harrah's Entertainment purchased the 30-year-old track last year for $73.5 million with five local investors who wouldn't speculate about how much revenue the casino will generate. In Louisiana, 18% of the slots proceeds go toward purses and breed development."It's really difficult to predict what our gross monthly income will be, simply because Harrah's has never before held ownership in a facility that also features horse racing," said Thomas Roberts, general manager of the track and Harrah's casino in neighboring Shreveport.(Harrah's is one-third owner of Turfway Park in Kentucky, but that racetrack doesn't have slot machines or video lottery terminals.)The Louisiana Downs casino will compete with five riverboat casinos in the Shreveport-Bossier City area, the state's most-lucrative gambling market because of its proximity to Texas. The ground-floor casino will have 903 slot machines. The track is constructing a 150,000-square-foot casino that will house about 1,500 machines. That casino is scheduled to open in May 2004.The first Louisiana racino at Delta Downs in Vinton fueled a purse increase from a daily average of $40,000 to $240,000.