I think you shouldn't start pulling weight until over 18 months, better yet after 2 years so you can get their hips checked to make sure they are OK for pulling huge amounts of weight. I know lots of people disagree, but I don't see why you want to make a dog pull real weight before they finish growing and maturing.
Getting the accustomed to the harness and dragging chains or small amounts of weight to get them ready is one thing, but actual weight, 2 years IMO
"There are two temptations to which people fall prey with dogs. The first is to think that biology counts for everything. The second is to think it counts for nothing"
- Tom Junod
“When you KNOW better you DO better.”
― Maya Angelou