I don't have any experience grazing chopped stalks. However I can see where chopping might make the lower stalk more palatable. And the lower stalk is where the danger of nitrate poisoning is greatest.

Making silage or baling gives the nitrates time to change form and is very different from grazing green stalks in the field prior to a hard frost.

If not chopped cattle will graze everything from the top down and only graze the lower stalk as a last resort and if you force them. If you are going to graze stalks I would not chop them.

Jim at Dawn