No, it's their only food, for caterpillars anyway. The adults won't lay their eggs on anything else.
The caterpillars feasting on the bitter milkweed sap is supposedly what gives the adults their unpleasant taste, which is why the usual predators want nothing to do with them.
Apparently, the sap doesn't taste all that repulsive. The squirrels ate the milkweed like they eat my tomatoes, cucumbers, eggplants, and just about everything I try to grow.
I'll have to try again, because I would like to have a friendly habitat for the monarchs. I get lots of butterflies and bees in my yard.