you can put as much as you want into them once there not to heavy, i have a deerhound x grey to a collie bull hound, runs good on all ground and im planning on putting him back to a weaton bull hound she runs well to an shes still a pup, that will be 5 different dogs in the pups, they both run well on smooth, rough and hilly ground, its not about how they turn out in looks its how they work and what they can do