Yes I don't think it is actually possible to harvest 100% of the field. 63% is a great score!
Initially, it was an intentional a design choice to have the edges of the field more difficult, so that it creates two opposing forces: To stay on the center is easier, there will be soon not enough crops on the center.
Of course, I realize that it is too hard in its current state. Probably, the enemy should spawn at least a little bit farther away, and/or the player should get access to a stronger means of defense as a reward after harvesting a significant %age of the field.
Thank you for trying the game and for your feedback :-)