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I presume that you are wanting to give the milk thistle for liver disease? The dose is about 50mg/kg of the active ingredient (silymarin) once to twice daily. So for a 2.6kg bird, her dose would be 130mg orally once to twice a day. You can mix with with the feed but remember this is the amount of silymarin in the product you are using, not the amount of milk thistle. Since there appears to be quite a safety margin on this compound, you can increase the dose to if you need to depending on the concentration of the product you are using. There is no evidence, however that more is better and that higher doses are more effective. The effectiveness of milk thistle in general has been questioned but it is not likely to hurt.
If you have further questions or need additional clarification please let me know.