I am looking for suggestions on how to properly condition my dressage horse to trot and canter on hard Arizona ground. There is a nice trail in a local park that I would love to be able to spend time working him at more than just a walk. He is barefoot and wears Renegades on the trails....would adding pads in them help with the concussion?
You would do it just like you condition for anything else - by gradually increasing the distance, speed and time spent on said trail.
Depending on how much (speed) work you're planning on doing, Renegades without pads will be fine unless your horse has super sensitive feet for some reason. But they should be tough enough if he is always barefoot.
If you're truly worried about actual concussion, the Renegades won't help all that much - it's just a piece of plastic after all. Instead consider "sneakers" (http://www.equithotics.com/sneakers.html) because there's actually an aluminum shoe inside a plastic compound. I have used them successfully for my endurance mare for the past three years and love them. But it sounds as if you want to keep your horse barefoot so this may not be an option for you.
actually, putting padding or metal shoes on the feet will increase the concussion- the horse's foot naturally acts like a shock-absorber. You put metal shoes on, you get a dramatic increase in concussion. If you put padding on, you mess up the horse's proprioception and he'll hit the ground harder because it feels "soft" to him from the padding.
Working on hard ground, as long as you don't overdo it, is quite good for their tissues- they adapt by making the bones and tendons stronger.