I’ve never had a horse who was allergic to alfalfa. But have one who breaks out in hives when I tried to feed it. A friend says that alfalfa cubes won’t cause an allergic reaction... just curious if this is true? 

Curious to know what would be different in hay and cubes. Cubes are ground and pressed hay, so . . . ?

Depends on if the horse is truly allergic to the alfalfa or, possibly the dust or mold spores in alfalfa. If it's the latter two, yes cubes or pellets could help due to the processing. Have you run an allergy panel to find out if there's an allergy and what it is?

I had one that was allergic to alfalfa. She couldn’t eat it in any form. You could try the cubes and see.

Ditto