You should probably add an option to turn on to use the mouse to 'shake' if it's possible and not too complicated. The concept of shaking a controller to milk a cow is interesting and silly, but also severely limits the amount of people that can play the game.
Thank you for the suggestion, I thought about this whilst making the game, however due to time constraints (I only had a week to make it) I didn't add it. I have been debating on how to add it, whether click and hold or toggle by clicking the udders. I am rather busy with a few projects at the moment, but I'll find time to see what I can do :))