As I mentioned it already, I mix 10-20g (1-2%) rubbed Oregano under 1 kg feed. I buy it in 1 kg bags. Additionally I mix 1ml (per kg feed) Oregano feed oil (I use the one with 2% for pigeons) under the feed - as binder for the rubbed Oregano.
Here are many ready to use oils and liquids, which are based on Oregano.
http://www.ropastore.com/us/poultry
http://www.ropastore.com/us/pigeons

As far as I know, the 2% and 10% oils are blended with Soy oil.
I don't know, whether it is GM-free Soy oil.
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