So, Im no dressage girl and certainly don't claim any intense amount of detailed knowledge pertaining to the way horses are built, but Id have to agree that the dramatic differences between a lighter boned horse (TB) and a heavy boned horse (DRAFT) would severely affect that animal's soundness over time.
The angles at the shoulders and knees also affect how well an animal can move within a certain discipline. Heavy horses have large angles at the shoulders and knees, connected to shorter legs for the torque needed to drag a heavy spade through earth. Their joints are wide, almost bovine-like to add surface area to support the immense pressures that hauling a load entails. Big butts and wide shoulders, though well-muscled, are still mostly bone. Light horses don't have the dramatic skeletal structure, but have finer and less 'bulky' bones for smoother movement and collection. Small shoulder and knee angles, coupled with longer legs and bodies (compared to a draft) give the light boned horses that movement we look for in dressage animals; the ratio of muscle to bone density is much different - less weight to move skeletally makes for a magestic dance indeed. To cross two pure-breds would be disastrous for the conformation, I'd imagine.
Most TB/Draft crosses aren't an F1 generation animal anyway. Breeding up to the ideal mix is something we've been doing to horses since we've lost the need for working equines (as opposed to eventing/sport equines).
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