A (semi-)serious article in the Mail Online discusses the possibility that cows have “regional dialects”:
http://www.dailymail.co.uk/news/article-401843/Cows-regional-accents-Pull-udder-one.html
This topic is very amusingly treated in a segment from the NPR podcast “How to Do Everything” (Dec. 31, 2013), where Sir Patrick Stewart comments on this phenomenon and performs various of the accents:
http://www.npr.org/blogs/codeswitch/2013/12/30/258393176/make-it-so-sir-patrick-stewart-moos-in-udder-accents