I have a smaller yard and two Rotties, so I am familiar with that outdoor urine smell. In the pet aisle one day, I spotted a canister of "Tidy Cat" sprinkles for cat boxes. I thought I'd give it a try. It has worked very well. The stuff is really potent, but safe for the dogs. I give the yard a good hosing and then sprinkle a can of this stuff around every month or so. It totally neutralizes the smell. I just make sure to hide the can from the girls. I don't think they would be pleased that I'm using kitty products in their yard.
Good luck,
-Camille