Organizers call it the Super Bowl of the dairy industry.  The World Dairy Expo opens in Madison Tuesday.  An estimated 70,000 people are expected to attend from 90-different countries.  Visitors from Canada, Mexico, Ireland, China, and Germany topped the list of international guests during last year’s five-day event.  The expo is a forum for dairy producers, companies, and organizations to come together to compete and learn more about technology and commerce.  The event, which runs through Saturday, features about 2,500 show cows.  Expo general manager Mike Clarke says Wisconsin’s dairy industry is worth more that $26 billion, making Madison the best spot to host the event for the past 46 years.