http://www.cbif.gc.ca/pls/pp/ppack.info?p_psn=212&p_type=all&p_sci=sci
"Brassica oleracea includes common cultivated crops such as kale, broccoli, Brussels sprouts, and cabbage.
All these vegetables are capable of forming toxic quantities of SMCO, a chemical that can cause hemolytic anemia in livestock. These plants also contain glucosinolates, which can cause goiter. In general, these widely used vegetables are safe for human consumption. Cases of livestock poisoning occur when they are used almost exclusively as fodder for animals (Kingsbury 1964, Smith 1980, Cheeke and Schull 1985, Benevenga et al. 1989). Glucosinolates contained in kale, cabbage, and broccoli (Brassica oleracea) can cause goiter in humans. These plants cause goiter in less than 5% of cases in humans. The chemicals cause a reduction in performance of young livestock, especially swine and poultry (Fenwick et al. 1989). "
A head of cabbage a week, split between some hens shouldn't be any problem at all. Heck, my boyfriend moved our chicken tractor over a poison hemlock plant I hadn't dug up yet. By time I noticed, it was all torn up by the birds. Not sure how much, if any, they ate..... but other than a couple of shell-less eggs (which could have been coincidental) I didn't notice any ill effect. And poison hemlock is pretty darn toxic. I see the list also includes stuff like onions, garlic, apples, buckwheat.....