If you chop them before you turn the cows out, any ears still out there will be shattered and scattered and the cows won't very effectively get the corn.  I'd just leave them, they'll eat all the husks and leaves and chew around on the stumps and stalks as there is usually quite a bit of moisture in them.  If you leave a bunch of cows on 10 acres of stalks all winter I doubt there will be anything left to chop in the spring.