At our Metro Area MNWWN gathering in October 2014, we discussed this topic.
Goats are being used in MN on many projects. Here is a link to a local SE MN company that rents their goats for buckthorn and other vegetation management needs: http://goatdispatch.com/.
Also, check out these links for more information on this topic:
Goats - Conservation Minnesota 2014: https://www.conservationminnesota.org/newsstream/goat-grazing-as-an-alternative-tool-to-invasive-plant-management/
Goats - River Bend NC 2014: http://www.southernminn.com/faribault_daily_news/news/local/article_38cad234-3694-571f-af92-f4c5070912df.html
Goats - Strib 2013: http://www.startribune.com/local/south/222820581.html
Goats - MPR 2013: http://blogs.mprnews.org/statewide/2013/08/goats-have-an-appetite-for-buckthorn-in-se-minnesota/
Hogs - North Central SARE: http://www.northcentralsare.org/Educational-Resources/From-the-Field/Minnesota-Producer-Experiments-with-Hogs-to-Control-Buckthorn