Here is a fix to use Alfalfa hay as hay in your mixer wagon
I fixed it, added DRYALFALFA_WINDROW to the "MIXERWAGON" filltypecategory
path: config/maps_fillTypes
And you need to add DRYALFALFA_WINDROW to the Filltype DRYGRASS_WINDROW under recipe "FORAGE"
path: config/animalFood