The first step to deworming pastures is to have your horses on a worming routine. Then removing or scattering the manure in the pastures and resting the pastures are very important. Pastures should be grazed on a rotating basis. Horses will not eat where they have peed or pooped which is why you can see pastures with really long grass and really short grass and wonder why the horses never eat the long grass. Go take a look and you will probably find manure in the long grass. Dragging the field with a chain or spike harrow can help rip up dead grass, aerate the roots and scatter the manure. If you know someone with cows or sheep run them through th epasture after the horses. They will eat the long grass. Do not run horses with sheep unless you have too, I have seen horses kill sheep by stomping on them in play. If you run cattle make sure you have enough water int he pasture as cows drink a lot more water than horses.
Over time if you manage your pastures properly and worm your horses correctly the pastures will become work free. Also if you get a new horse do not turn them out onto the pasture until they have been wormed. Keeping them in after worming so you can inspect their feces to see if any worms passed can be helpful.