I only recently read about the cats preferring the non-red-stemmed variety, just when I had collected a LOT of it to bring home, cut, in a pail. They've eaten it fine but another variety of milkweed with smaller leaves and white flowers has been totally ignored by the cats. Too bad, as it seems to last longer in water.
We've discovered the best way to preserve leaves until needed is to cover them with a wet paper towel and keep them in the refrigerator.