We have two cobs who are not too shabby in the old dressage arena
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I know little of these things, but Back Man was saying that cobs are bred and built for pulling the weight into the shoulder area; their hocks/backends are not designed/built for the action and power needed for the higher dressage movements.
I'd love Little Cob or Big Cob (or any other cob for that matter) to prove the exception