I use the product Cushing's Care from Herbs for Horses. it is a Canadian product so I realize not likely the one you are using. anyway, the main ingredient is chasteberry and I think they only add some Omega-3s to the mix. their product is pharmaceutical grade which is important to check regarding the one you are using because that will affect how much to give and the quality and therefore the efficacy. if you can find it get pharmaceutical grade.
they recommend 15 grams per day per horse. I've had my mare on it for 4 months but the company says not to expect complete results on efficacy for 6 months. I had my horses ACTH level checked in late August and I am going to have it rechecked again in late December. if I don't see any change then I will likely put her on pergolide. but the company says they have had great success with it over the 10+ years they have been manufacturing it.
good luck. I always think with horses the herbal route is worth trying.
Laura & Hope
Kelowna, BC, Canada