I was concerned about feeding them hay, as I have heard so many stories about impacted crops, and most of them seem to involve hay or straw or grass. But many here have given them hay, and they really enjoy it, many others have free-range chickens that have access to hay. So I do give my girls a flake of alfalfa every other day or so. It really gives them nice orange yolks, and keeps them occupied. I make sure they always have other food available, and plenty of grit, and I have never had problems (@ 6 months?). So it is pretty much up to you whether or not you feel it is safe.
I feed my girls top quality horse alfalfa and they pick around the stems and eat the leaves. I've not had a problem with impacted crops or the like. They also get the dollar bag greens from the store so that it's not their only source of vegetables.
I have six birds, bought this bale of hay in November and it's just now almost gone. It doesn't take much to keep them happy.
I do have a con that comes with my hens eating alfalfa hay. That being that the hay is in the pig pen and sometimes the chicken jumping in to get a mouth full sometimes gets mistaken for the pigs lunch.