Horses and Allergies

My husband has standardbred race horses and sometimes the horses have allergy problems. He has heard that there may be herbs he can add to their feed to help with this problem. Do you have such herbs or even know what they might be?

This is difficult to answer without further information. Are you referring to allergies that affect his horses via the respiratory system or on the skin (allergic dermatitis, etc) or in some other fashion?