It's not a threat or a joke. Its a sign! Something including cows is going to happen in the future, and knowing how aliens abduct cows a lot... Aliens will invade Blockla---ehh, too impossible. Probably, just a sign that ol' Eric was thinking about America's best meat product, but wasn't in a good mood. But I don't get why other people didn't see it.
Offtopic: Anyway, there's a chiropractic somewhere around Ottawa county(Michigan), and his name is Eric Hartman O___O A saw a business card with that name on it and the words Chiropractic Professional on it.