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I do feed bok choy. And I spent 2 years researching for a book I was writing --- granted it is about finches, and things are sometimes a *tiny bit different* (pun intended) but in all the loads of books, books from my vet, and articles in Veterinary Journals and the like -- I never read this.
Cabbage contains Glucosinolate that can be toxic in high doses. (to both human and animals) But I don't know if that can cause esophageal tumors.
Broccoli also contains Glucosinolate so I only give mine that 2-3 times/week.
Their is a lot of things that are toxic in high doses (spinach and clover for an example) so to eat a varied diet every day is the best.