The bulldozer is your second best friend (second only to trashing contracts.)  This is where you can start to get really creative... Just expanded and low on feed?  Build a house and a field, grow some crops, then bulldoze them. Co-op running corn? Take out your second and third chicken and put in an extra field?  There are some farms that have a couple of everything and then have space that changes based on current need.

*** THERE IS NO UNDO BUTTON ***

You can not bulldoze a building that is running, so always start what you want to keep before you start bulldozing.  This habit will save you some late night pain.