I am noticing the same shedding issue in my mare Bahj-Rha. She is a 28 year old Morab. Currently sher coat is longer than it should be and last spring and this spring she blew her coat in early March. She was darned near nekid! The hair growth came back to its almost winter thickness and she is now shedding heavily again.
My vet has never heard of Chaste Tree Berry but didn't think it would cause her any harm to try it.
Her other symptoms are really mild so we are not considering pergolide yet.
Marion and Bahj-Rha