Not so sure about spreading the calving over a wider time-frame....

Not so sure about spreading the calving over a wider time-frame. I assume you're thinking about improving the milk production outside of peak periods? Increased feed costs could outweigh any perceived benefits, then there may be animal health issues. Calving generally occurs either in the autumn or spring in SE OZ to take advantage of peak pasture production. Also, having an even line of weaners for sale is far preferable, and better financially, than trying to sell a mixed bag (age wise). And then there are the animal husbandry costs: far more cost effective to deal with all the calves and cows in one hit, rather than multiple times a year. IMO