My three year-old daughter is still working on some consonant sounds. She has yet to master "th" and substitutes "F"s instead. When she stepped on a thistle yesterday evening she cried, "I stepped on a fissile!" As painful as that can be, I can barely wrap my brain around around trying to eat them, yet there are people who claim great success with training livestock to eat "weeds", including quite barbarous plants like multiflora rose and canada thistle.
I have some patches of thistle in my pastures. Last year I thought I'd take a crack at training some heifers according to the techniques on the linked to site above. I can't claim great success with getting my bovines to eat thistles, though the training program does seem to have broadened their gustatory horizons because now they demolish every burdock plant they can find, as well as milkweed, both of which were only subjected to occasional nibbles until last year.
A few weeks ago I found another variety of thistle growing in a hay field and thought - "aha!" - the spinyness of thistle varies hugely. I managed to delicately nibble down a leaf of the hayfield thistle (the top plant in the lead photo). I don't even like walking through my pasture thistle patches let alone chew on them. If the trainees get to munch on leaves with few spines they can easily learn to eat "thistles".
I resorted to brush hogging the worst of the thistle patches because it allows the grass to compete effectively. It doesn't eliminate the spines from the pasture, but it reduces their prevalence significantly. Four years ago I had a patch that I mowed about three times in two years and there are very few thistles in that spot any more. I think mob grazing (trampling) the thistle at 500,000-1,000,000 lbs liveweight per acre for a brief spell would be as good as mowing, but I didn't get my act together with that this summer.
This is not to say cows and pigs can't/won't eat spiny things. I know enough people who have seen it happen to discount the possibility. I just haven't observed my animals choose to eat the really really spicy plants...