Pak Choi belonngs to the cabbage family. I wouldn’t recommend going to the store and buying it. However, it would be OK if you already having it to eat and feeding only a small tiny amount as part of a mixed diet for your Herman tortouse.
Note: It contains other compounds that can negatively impact kidneys, and as such is not a good general food.
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Can Hermann Tortoises eat Bok Choy?
Bok Choy is fine for your tortoise when fed in small part of a wider, varied diet.
The plant has goitrgens, that can impact the thyroid activity and contribute to bladder and kidney stones.
Like any food/feed, give only in moderation. Remember, to provide adequate water when feeding to keep things balanced.
You shoukd use all plants in moderation.
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Bok Choy, can then be used once or twice a year with no concerns, as is the case with almost all other edible plants out there.
I also recommend keeping a cuttlebone in his or her habitat so the tortoise can self-regulate calcium intake and other trace minerals.