One copy of this is found in the copper-etching cabinet in Dresden (Kupferstichkabinett). There it is titled 'Der Mönch im Kornfeld':
http://skd-online-collection.skd.museum/de/contents/show?id=559514
'Kornfeld', which can be translated as 'corn field' is never used to refer to a field of corn=maize in German (that would be 'Maisfeld'). One would expect wheat or rye. Grimm's lexicon has the following to say:
kornfeld, n. feld mit getreide oder roggen. Frisch, engl. cornfield, nl. korenveld:
'Kornfeld', n., field with wheat or rye, and it seems to directly correspond to english 'cornfield'.
The ears also look more like wheat than maize/corn to me, but I'm no art historian myself. I also suspect the association of corn with maize is rather modern and definitely older younger than the word 'corn' which goes back to indo-germanic roots.
Maybe this helps a bit.
There is no consensus on exactly when corn was introduced into Europe, but it was definitely in either the late 15th century or very early 16th century - so at least a century before Rembrandt lived. Corn, which is native to the New World, was already found in China some thirty years before Rembrandt was born, so it was definitely around. Whether Rembrandt would be familiar with it, I cannot say. I suspect he would have been, given where he was from, but do not have any sources to back that supposition up.
In the book Maize and Grace, which is about the spread of maize in Africa, the author notes that it spread so quickly, that many instances found European explorers encountering African villages (in the interior) for the first time in the 18th and 19th century and finding maize fields. The author speculates that the corn seed was traded by Europeans to Arab traders, who then brought it down the East coast of Africa, from which it reached the interior.
Also, what /u/Astrogator says below is correct. The original Dutch title is De Monnik in het Korenveld which actually translates to "The Monk in the wheat field". Koren, which can translate to "corn", is actually better translated as "grain", in this case, specifically wheat.
