Our feed merchant told me it's the other way about, Fleecewife; the manufacturers have to put an additive in to make the feed suitable for tups - and they don't in all of the feeds, sadly.
Reading up, the formation of urinary calculi (stones in the bladder or ureter) is more likely when animals are fed a high phosphorus diet, and less likely if the diet is higher in calcium than in phosphorus, preferably about 2:1.
As far as I can tell from reading up, all animals (both sexes, sheep and goats too, apparently) can be affected, but clinical symptoms are actually most likely in early-castrated wethers, because their penis is less developed, so there's less room for a stone to exist and not cause a problem.
However, it seems to be only recently that sheep feed manufacturers have realised that they need to add extra calcium to store lamb feeds as well as to feeds formulated specifically for tups, as the condition can affect store wether and tup lambs just as easily - or moreso - as it does adult, working tups.
So for the moment, unless the packaging says 'suitable [or unsuitable] for tups / rams', I think you have to ask the manufacturer directly in order to be sure. I asked the supplier we use for our lamb cake, and they had to check to be sure.