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I would probably avoid organic material such as sawdust.
1)Grass will likely grow in the road.
2)Depending where you are, sawdust and moisture, when/if it freezes, it can take a much longer time...

I have bottlefed. The only issue you have mentioned that is different than my experience is your calve is running loose. Mine were kept in a barn, otherwise I expect they would have knocked me down...