I was checking my search logs -- looking at what search keywords people use when they find my blog -- and found that someone googled "why can't quarter horses do dressage?" An interesting topic! And I guess the short answer is, "They do!" Here is a picture of Patrick Marley and his Quarter Horse mare, Honey Bright Dream. The mare is a stock quarter horse with no thoroughbred blood. The pair earned the Gold Medal in their first season at Grand Prix.
When you talk about what breeds can do upper level dressage, you make broad generalizations and think about what traits most of those horses possess. But there is an awful lot of variation even within a breed or registry. The AQHA permits the infusion of other bloodlines so long as they conform to the breed's standards. You can find quarter horses that are stock type, some that are hunter type, etc. As I have heard so many times from experts, "look at the individual horse standing in front of you."
Quarter horses in general
From the standpoint of temperament, quarter horses have a laid back and forgiving nature. While it sounds like weak praise ("you have a nice personality"), it's actually pretty important. Barns are full of fancy warmbloods that top out at second level, either because they find collected work distasteful or because they lack the right trainer to get them there.
What keeps quarter horses from moving up the levels?
Here's a good guess from a non-expert: Conformation. There is a lot of individual variation in the conformation of a quarter horse, depending on what discipline they compete in. There seems to be a fair amount of thoroughbred blood in at least some quarter horse lines. The desirable traits in a "classic" western quarter horse include low head carriage, a rather straight hock, more upright pasterns, blah, blah, blah. To me, it's easiest to just look at the typical quarterhorse outline (see above). Does it not just SCREAM downhill?
An important quality to look for in an upper level dressage horse (assuming you can pick just one) is an uphill frame to facilitate collection. While there are downhill horses that have made it to the upper levels, it's more the exception than the norm. If I were looking for a quarter horse just for dressage, I'd look for a horse from a breeding program that produces hunter or even dressage types. My Royal Lark is a great example.
My Royal Lark in a dressage test
Indian Artifact and the DeGraff Stables breeding program provide another example of quarter horses suitable for dressage. An Indian Artifact son is shown here:
A Quarter horse from the Niobe Valley Ranch...
When all's said and done, I see a lot of quarter horses competing in the lower levels, where most of us are at in our riding anyway. They may not all be huge movers but they can really shine in areas like obedience/submission, relaxation, elasticity, acceptance of the bit, etc. I've seen some nice ones in the ring, esp. the appendix registered QH. Even those not destined for Grand Prix can be star mounts, especially for the younger kids/juniors getting started or the adult beginner rider.
American quarter horses in dressage from Equisearch
Quarter horse dressage
New program recognizes quarter horses in dressage
Niobe Valley Ranch dressage quarter horses
Golden horses of the Niobe Valley