Astragalus: Can It Be Fed to Animals?

I work in Research/Extension and am in the process of trying to identify some new agricultural crops for production in the northern U.S. plains (semi-arid climate).
I have read that the root of Astragalus membranaceus takes approximately 3 to 5 years before it is ready to harvest for medicinal use. I have also read that other Astragalus species are grown as forage. My question is: can A. membranaceus be grazed or hayed during the years leading up to root harvest, or will this comprimise the quality of the root?

I asked my friend, Zhongfa Wu, a Chinese herbs expert based in Canada, to check with his contacts in China for the answer to your question. He says that huang qi (Astragalus membranaceus) is sometimes fed to animals in China but this practice is not common. If the the foliage is cut for hay then it is better to do that in the autumn and not during the summer. Summer haying or grazing will negatively affect root development.
Zhongfa adds that Chinese licorice (Glycyrrhiza uralensis), another medicinal root crop in the same family as astragalus, is commonly fed to animals as hay without negative effect on the roots. So your idea of enhancing the value of perennial root crops this way has some merit and is an idea worth further study.

Secondly, we are zone 3/4 here. Will A. membranaceus have trouble surviving the winter?

At this time I cannot say if the crop will survive winter in your area. Huang qi is a northern Chinese species but so far our information suggests that it is hardy only in zones 5-8. However this rating is still a preliminary one because the crop has not been fully tested in all zones as far as I know.