That is well before the Columbian Exchange, right? So how can they be growing maize in the Jin Dynasty? Is it some sort of old world maize or is this just an oversight by the author/editor? If it is some sort of old world maize, how does it compare to modern corn? Is it still around, who ate it etc.?
I'm somewhat suspicious of this book because I've found a handful of typos (one really funny one turned, "...people of Han." into, "...people of Ham.") and I'm wondering if there was just some bad editing going.
Your book is a translation, yes? To help someone to reference the German original, could you provide a chapter/page reference? Just a guess, but "maize" makes me wonder if this is a translation error: perhaps a truer translation would have been "corn" (UK usage) or "grain" (North American usage).