William and Louis eat it pretty much every day out in the New Forest. You can't avoid it with around 4,000 ponies living there
It seems to appeal to them more during spring and early summer, a bit less so during winter. The New Forest ponies are drifted (rounded up) once a year in the autumn and some of them are wormed then, but some are never wormed. I know when my local area has been drifted and walk the dogs elsewhere for a few days afterwards just in case they're attracted to horse poo with wormer in it, but they don't seem to be.
Lots of people ride in the New Forest too and I expect their horses are wormed regularly, but unless I were to muzzle William and Louis on walks it's a risk I have to live with.