To set up a herd so that it is not attached to any fields:
Open the livestock list
Tap on the Herd
Tap on the herd icon you want to remove from the field
Check that New location says "No location"
If you delete a herd then you will also remove all of the herd and history as well. We are adding an archive feature for herds this year that will allow you to archive herds but keep the history information saved.