Steve, You don't necessarily have to redefine the field. If you go under the History menu to Chemicals and select Add/Edit Records you will go to the Edit Chemical Operation box. In this box you will be able to select the desired field (even though it has been deleted) and add a chemical record to it. If you want to update the Notations also you can go under the History menu to Notations and select Deleted Fields. You can then select the desired field and add notations to it.
You're idea of redefining the field and recording the farming action then deleting the field again will also work.
If you have a field listed twice, I would try the following:
1. Redefine both fields. 2. Find the field with the fewest number of farming actions and print the notations out to a printer. 3. Rename this field. Give it a name such as "Delete". 4. Go to Remove Farming Actions from the Setup menu and remove all the farming actions from the "Delete" field. 5. Go back and check the field and make sure it no longer has any notations. If any remain, delete them. 6. Delete the field and the name should be gone for good. 7. Check the notations for the field that remains. If any of the notations from the deleted field are missing, then re-record the farming actions.