6000 liters per ha is the threshold I would put to go forward.
A pleasure I therefore claim to show, not how men think in myths, but how myths operate in men's minds without their being aware of the fact. Levi-Strauss, Claude
But it's a winter crop, fits into a rotation and is not therefore usually monocropped. There's a GM danger, (it has been considerably worked on by Monsanto et al), but otherwise it could be more sustainable than maize or sugar beet.
