Richie Sambora -- Breeder's Cup BARRED ME from Playing Hardcore Guitar Riff

Richie Sambora was NOT allowed to play a hardcore guitar riff during the Breeder's Cup because officials were afraid it would cause the horses to FREAK OUT ... and TMZ has the rendition that was too hot for the race track.

We've learned ... after Sambora was tapped to play the traditional Call to Post at the race, he whipped up two versions to submit to the race officials -- a heavy metal riff and a more toned down version.

Race officials tell TMZ ... they loved both versions, but were afraid the heavier riff could spook the horses -- so they nixed that version and made Richie go with the more mellow rendition.

Fortunately, things went off without a hitch -- Sambora rocked his performance (none of the horses went crazy) and race officials tell us they want Richie back next year.

In fact, race officials tell us ... NBC has already expressed interest in using Richie's version in horse racing promos in the future.