It is true any line of dogs can be taught to pull. I took a scatter bred do of none prove pull dogs and went on to place in every event I entered him in. With that being said you can not replace a dog from prove pulling lines mostly if the breeder of said line is trying to produce better every time.
I took said Dog on a winning streak of straight first places in to MO and pulled against a couple last name Vallines. They pulled their dog fist two weights and as three rest of us pulled three or four more time they sat the dog out waiting for more weight. when all but two dogs in my class were left the put their little dog back on the track and he pulled three more time after the rest of the dogs in his class could not. they have been perfecting their stock for years and a dog not bred for weight-pull would be hard pressed to beat them.