The Collaborative International Dictionary of English v.0.48:

Mangel-wurzel \Man"gel-wur`zel\, n. [G., corrupted fr.
   mangoldwurzel; mangold beet + wurzel root.] (Bot.)
   A kind of large field beet (Beta macrorhiza), used as food
   for cattle, -- by some considered a mere variety of the
   ordinary beet. See Beet. [Written also mangold-wurzel.]
   [1913 Webster]
   [1913 Webster]

WordNet (r) 3.0 (2006):

mangel-wurzel
    n 1: beet with a large yellowish root; grown chiefly as cattle
         feed [syn: mangel-wurzel, mangold-wurzel, mangold,
         Beta vulgaris vulgaris]
    2: cultivated as feed for livestock