I've not used global herbs before but when my horse had horrible diarrhoea I used this and it was amazing! http://www.naturallyanimals.co.uk/naturallyanimalsshop/prod_1744934-Naturally-Healthy-GastroIntestinal-System-15-10-kg.html
One you can definitely grow without trying is nettles. My horse loves them :)
Also if you have any dandelions in your lawn (if you have a garden), they love the long root as well as the plant on top and it's very good for them. You can also go around the countryside and collect rose hips from...