Also I would switch your Fed to Date calculations around so the current total adds the Today values BUT see last comment below.
Can provide better help by seeing a screen shot of the table structure.
Should look something like this:
FedTableName
[ID]
[ProteinName]
[ProteinFedToday]
[ProteinFedToDate]
[ProduceName]
[ProduceFedToday]
[ProduceFedToDate]
Where [ProduceFedToDate] is a new total for the record based on all previous calculations. Another way is something like:
FedTableName
[ID]
[ProteinName]
[ProteinFedToday]
[ProduceName]
[ProduceFedToday]
Me![txtUnboundTextBox] = SUM(FedTableName.[ProduceFedToday])
Where there would be no saved value in table for [ProduceFedToDate] and the same applies to [ProteinFedToDate].
I surmise you are having an error because as part of your calculation is the current field value, which may be null. Therefore if I tried adding something to a blank field (i.e. itself - [ProteinFedToDate]) I would have either just the addition ([ProteinFedToday]) or nothing at all because the system will be confused.