A Question-Cattle and Minerals

So here's my question.  I would like to be able to raise our beef cattle from foods we've grown here-if we have to.  I realize that I can't get my own salt for them, but what about mineral supplements?  What vegetables and grains would be wise to grow that would include the minerals that the cattle are now getting from the loose mineral we purchase?  What did Pa Ingalls give his cows?  Our soil is lacking selenium here.  Would the grains and vegetables grown in it be lacking selenium too?  Is there a way to add selenium to the soil?  Any extension agents out there?
Thanks...