Hurro...
For conventional stock you will need a model release for the jockey, and if the horses have visible brands that are obviously stable brands (as in burnt-in tatoos, not branding on harnesses etc) you may need a property release for the horses. For editorial you obviously don't need any, but I suspect editorial horse racing photos may not sell that well in the scheme of things.
I have some shots of harness racing as general stock where the faces of the riders were obscured and I did substantial editing to remove/obscure sponsor logos and horse brands. I've also got some I'll put on SS as editorial at some stage, but havn't got that far through my 'must edit and upload' folder.

For what it's worth, they don't seem to be stellar sellers, but one of them has done OK on all of my sites:

Because racing is a logo rich environment, with everything from the tack, buggies, sadles, covers, numbers, fences, railings, jockey's clothes and even the horses themselves in some way marked with potentially copyrighted logos I personally think the of the level of editing required to make the images acceptable RF stock is too high to make it economic in pure money terms. ie: unless I luck in with a logo free race image when I'm covering the races for the paper I wont be submitting any more of them to non-editorial stock agencies.
Cheers, Me.