The latest requirements for Wheat Crop are as follows:-
Harvests in - 12 Hours
XP gained - 1
Cost per field - 13
Yield per field - 61
Mastery Stars at 500, 1500, 3000 fields
You will have NO CHANGE in mastery, if you have already mastered Wheat Crop.
The old data for Wheat Crop can be seen in this picture:-
"Please note: Due to technical issues, this feature is not yet available. We sincerely apologize for any confusion or frustration this may have caused you. We will do our best to get this feature to you in the near future."