Like a lot of others on here my Ben had pancreatitis when he was young, and it has left him with a tendency to small flare ups if he eats too much of the wrong stuff or anything that is high in fat (like bird fat balls as we learnt from experience!!). We keep him on a low fat diet and he is fine. He is also a terrible scavanger, we learnt the reliable recall as a means of calling him away from anything found on the floor, and started taking out really yummy things he can eat like warm chicken or tuna cake.
We can now call him away from most things, but every now and then he finds something before we see it, and often this does lead to an upset tummy, however unless he had a life threatening illness that meant he was in extreme danger I would not choose to muzzle him, it would spoil my enjoyment and I am guessing his too. I know that there is an element of risk to this approach as a very severe episode of pancreatitis could potentially be fatal
but for me personally the sort of stuff found in the woods dosn't usually cause an issue. I have to say I didn't realise horse poo could cause an issue - I always assumed it was just high fat foods as that is what triggers Ben off, so that is useful info to know.
hope you find a solution.