Many students choose a topic that they already know something about and explore new research in that area. Whatever you choose, make sure you are interested in the topic!
If you don't have an idea yet, browsing through the library's book collection in animal science will present you with a broad variety of subjects. You might also want to scan issues of the Journal of Animal Science or the Journal of Dairy Science to see what scientists are studying.
A quick way to get a feel for how well your area has been researched is to run a search in Google Scholar. If only 10 hits are returned you'll know you have to expand your topic or change your topic. On the other hand, if you get 10,000 hits you'll know you have to narrow down your topic.