Cargill's HQ is maybe 15 miles from Minnesota Carlson. It doesn't get any more "agribusiness" than Cargill.
Agribusiness seems to be relatively underrepresented in graduate business schools compared to the size of the industry, even at Midwestern schools that have undergraduate agribusiness or applied agronomics programs. If you can't find a business school with the specialty you want, you can at least be close to the action in terms of internships and networking.