Hmm I have some bad news for you :( As long as there's grass to eat, the speed at which cows produce milk can't really be increased. Clearing the Dung will help make sure there's grass to eat but you can't put fertiliser on it to make it grow faster and even if you could, it wouldn't affect milk production.
Other players have noted that there aren't enough cows in the world to make milk fast enough to keep up with the demand for pies. This is something we need to balance properly in the future.
Thanks!