This is a subject I have some very strong opinions on. The actual weight of a newborn is relative to so many factors it's not even funny. Caving ease is what I measure. If you have been single (traiting) for birthweight, your ease to weight ratio will be much differant than ours. Here we have all but eliminated assistance in the adult cow herd, but we still see some 100+ birthweights. I still expect up to 100% assistance on first calf heifers because of the psychology (inexperience) of the animal. "Calving 1 heifer, have 1 calf puller".
Energy intake is often over looked and is one of the easiest management tools available, accounting for up to 33% of calving issues.
Environment accounts for another 1\3 of birth issues, can't be controlled, but can be expected/factored in.
So you have probably figured out by now I only expect the bull's genetics to be 17% responsible for calving problems. Don't blame the bull! But he can make the difference.