I have been feeding alfalfa to my chickens for several months as an occassional treat. They do love it and work hard to strip it right down to the stalks.
My birds have been free rangers since they were just little tykes. In fact I got them in the spring when I also had a terrible infestation of chick weed in my garden.
I would take in big hand fulls of chick weed in almost every day and chop it up for them. They loved. Still do except now they can go get it themselves.
When they were about two months old I extended their pen into a garden area that had butter cups and foxglove in it. Both of which are supposed to be poisenous but they seemed to eat measured quantities without ill effect. They had lots else to eat but they wanted butter cups.
They have always had good access to grit, and I've noticed they eat a lot.
Right now I am buried in snow and they get grit under the coop they live in.
I've never heard of the alfalfa cubes. I will see if I can get them locally.