Hay is always going to have value... if things go completely south, the folks with horses will be the only ones travelling very far. I can think of lots of things I'd barter for good hay.
On the other hand, wheat is good... thing is, do you have the equipment to harvest it yourself, or would you have to depend on someone else? I know wheat can be harvested by hand, but that's real work.
If you were to break up your alfalfa fields and planted wheat, you'd have to replant the alfalfa if you wanted hayfields in the future. I'm not as familiar with alfalfa as I am southern grasses... but can't you plant an alfalfa field, and only have to fertilize each year (not replant every year), to get cuttings. So, if TEOTW arrived, and you had alfalfa, next year you'd still have some alfalfa growing... plant wheat, and next year just native grasses. Some alfalfa would be better than some grasses...