Does anyone know anything about horse racing?

[plug]

What does "needs to be supplemented" mean when you're looking at the odds on a horse for a future race?

[TicTacsMum]

The owners need to pay extra money to include the horse in the race. They will already have paid an initial entry fee.

Apparently!!!

In the derby the supplement can be up to a million quid!!!

[plug]

Ooh thanks, this is for the Oaks, so suspect it'll be a few bob too!