You are right to be worried about urinary calculi in male sheep, most especially wethers (because they have a narrower urethra than entire tups). If you must feed them, then make sure it's a feed designed for tups, not just a general sheep feed. If you do have a general coarse mix, then pick out the crushed peas and maize, and feed a couple of bits of that to your wethers, very occasionally. Really, hand feeding male sheep, whether wethers or entire males, is best avoided, as they can grow up very pushy towards you, and with a big sheep like zwartbles that can be dangerous. Unless you live somewhere very cold and snowy, keep the treats down to an absolute minumum, one or two small nuts per ewe. In deep snow, we feed our tups Carr's Champion Tup, and we feed that to the ewes too leading up to lambing, as our breed doesn't need 18% protein, so the tup mix is fine.