So you're wanting to cut off that east part? And have it re-work or somehow redo the zones after that? 

Easiest would be to tell the agronomist to fix their field boundary, redo zones, and send you a correct file. But...yes you can cut yours down. But it's not going to adjust the map you're left with. It will just keep what's left like you chopped off that chunk w/ an axe. So your lowest zone will still be right along that edge and the rest of the map will not change one bit. 

If you have your own yield map it looks like, and you like that...just use that to make your zones. Start w/ that, turn it into a grid or contour map. File>New>Generic or Prescription and go from there.