The question to ask with sheep is not “what do I worm them with” but “HOW do I PREVENT worm load?”
In the organic world, you need to prevent problems, because treatment “can” be slower acting than chemicals.
Here’s some of what we do:
1. We feed in a feeder in winter months – not on the ground. When we shear, we just throw away the strip of wool along the neck that gets matted from them leaning into the feeder. (As my husband says, we “cut out the watermelon” meaning we keep the best part of the fleece for hand processing or selling and use the leg, neck, belly wool around the farm for other uses.)
2. Salt and minerals – we always have free choice Redmond selenium salt and Purina Sheep minerals available free choice. We have started adding a little Diatomaceous Earth (DE) to the mix and add a little Apple Cider Vinegar (ACV) to the water trough.
3. Rotational paddocks – The sheep are never in the same area in the same year. You don’t have to be that drastic if you have good pasture, but we’re still building ours up. We also rotate with the cattle, so the sheep go in and eat down the early pasture and then the plush re-growth goes to the cows.
4. Garlic – You can try to feed your sheep garlic as a treat, sometimes they will eat the clove right out of your hand. If not, you can crush it up into a bolus or pop a clove in the back of their mouth and hold their head up so they swallow instead of spitting it out. We have also tried a mixture of garlic juice, molasses, and water as a sheep drench (see natural dewormer resources below).
5. Apple Cider Vinegar, diluted, can be a good boost if an animal is sick: http://www.sheepmagazine.com/24-1/laurie_ball-gisch/ and http://www.willwinter.com/#!Apple-Ci…5-B4C9BD0AD110
What people say on here is correct in that if a sheep has a heavy worm load, yes, it can kill them MUCH QUICKER than it would different species. I had a ewe with bottlejaw and we effectively treated her to where it was gone within 24-36 hours. She came from a commercial farm and was a sort of “rescue” sheep, and her immune system doesn’t seem to be as strong as the lambs we purchased and raised ourselves and now our own lambs born on the farm. In the case of a sick ewe or lamb, chemical deworming may be necessary to reduce the heavy load, then maintenance with natural products may be sufficient. For us, it’s a work in progress since we are new sheep and sheep management!
Natural dewormer resources: