Your chickens will LOVE you for giving them some forage. Alfalfa will be fine, buckwheat may not be as good.. ANimals can tolerate it in small amounts, but as I understand it, buckwheat causes problems with resisting the SUNS effects?
Even just grass clippings from your lawn mower will have your chickens running about pecking like crazy.. Yes, you can give them alfalfa cuttings.. Come fall, I often give them corn, beans, Tomatoes, and everything that's left over from the garden,and they eat it like candy!!! Egg laying usually gets a boost too!