09/22/2004 11:00PM

Top threats in derby each come with a catch

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ALTOONA, Iowa - The three main contenders in Saturday's $75,000 Prairie Meadows Derby all have questions to answer.

Knox, the 2-1 morning-line favorite, must prove he is as good on the dirt as he is on the turf; Polish Rifle, the class of the field, must overcome a 7 1/2-month layoff; and Thermostat, an open-length winner in each of his last two starts, will be tested for class in the 1 1/16-mile derby, which drew a field of six 3-year-olds.

Knox enters the derby off a second-place finish in the Paradise Creek at Arlington Park two weeks ago. In that 1 1/16-mile turf contest, Knox rallied from seventh a half-mile out and was beaten by 1 3/4 lengths.

Knox's only start on a fast dirt surface this year resulted in a 1 3/4-length win in the $75,000 Regeay Island Stakes at Ellis Park on July 17. That race, however, was an off-the-turf event that had few with proven dirt form, and his task figures to be much tougher Saturday.

Knox has a career record of 3-3-3 from 13 starts good for $140,957 in earnings, the highest in the field. Carlos Marquez Jr. picks up the mount Saturday.

Polish Rifle, trained by Steve Asmussen, finished fourth in the Grade 3 Risen Star over the winter at the Fair Grounds. Polish Rifle went to the shelf after the Risen Star and makes his return to the races Saturday.

Asmussen has put a string of solid works into Polish Rifle at Churchill Downs over the past two months, including a six-furlong bullet move in 1:12.20 on Sept. 6. Tim Doocy, the meet's leading rider, picks up the mount.

Thermostat is coming off back-to-back wins at Arlington and Turfway. He led throughout in both of those races, the most recent of which was a first-condition allowance event at one mile two weeks ago.

Trainer Rebecca Maker shows confidence in Thermostat by stepping him up into the stakes ranks Saturday. Frank Lovato Jr. will be in from Chicago to ride Thermostat.