Before I attempt to give my opinion and try to answer your question, I'll ask a question of my own: Why are you wanting to hold some of your cows over to calve in the fall? I'm assuming for 1 of the following reasons:
Said cows are "stragglers" at the end of the calving season and you want to move them back to the front of the next calving season
OR:
You've never had a fall calving season and would like to give it a try.
If there is some other reason, please fill us in. It might make a difference in the answer you get from some people.