I've posted on butter before, but this article is the best I've seen, with the most info. The text is very informative, but there are also simple and clear graphs, a table, and graphics, without which the article isn't nearly as good. Fi, there's a graphic showing which butter brands to buy, or not, and why.
If you don't already eat butter, go get some, and toss your margarine. And no, don't feed it to the dog; it'll harm his health, too.http://www.lewrockwell.com/2014/03/joseph-mercola/the-butter-boom-is-on/