"Peter has been preoccupied shredding mangel-wurzels "

Mangel-wurzel
Creative Commons Attribution Share AlikeMangel-wurzel - Credit: MarkusHagenlocher
Mangelwurzel or 'mangold wurzel' is a root vegetable used to feed livestock, although it is also suitable for human consumption.

It comes from the German Mangel/Mangold, meaning 'chard' and Wurzel, meaning 'root'.