You need to talk to your vet or an SQP. It will depend on the horse's age, weight and history. And on what your Yard requires. It isn't as simple as worming them and chucking them out in the field.
If you want to cover tape worm and encysted red worm without using Pramox, I was recommended to use Equitape first (stays in the system about 48 hours apparently) and then a week later use Equest which is longer lasting. It's the same drugs but in two doses and thus is less of an assault on the system.