horse racing riddle

This riddle is another of my favourites, this version is taken from,

http://theweeklyriddle.blogspot.com.au/2009/12/horse-race.html

ummmm.....I guess the humour will come from the witty remarks of HC posters....

The 25 horses at the Belleville Race Track always finish according to their rank: the champion horse wins every race; the number two horse always finishes before everyone else, but loses to the champion; number three loses only to number two and champion and so on - the runt always finishes last.

All of this was well-documented until the race officials lost all their records and time-keeping devices in a flood. This was bad timing because they need to send their best three horses (ranked in order) to the National Championship as soon as possible. All the horses look alike, so they need to hold a new set of races to re-establish their best, second-best, and third-best horses. Their track can only hold 5 horses at a time. What is the fewest number of races necessary to establish first, second, and third?