Howard's theory is popular, but I don't think there is a simple answer. Six of mine are doing it with new grass but none of them have any stomach upset or need worming They just enjoy it, carefully picking the freshest and youngest shoots. The three wolfhounds don't seem to bother, but they have their own preferences. The old one, in particular, has been known to pick out particular plants which the herbal says are beneficial in aiding digestion, so maybe she knows someting which our ancestors knew.