[el_oni]

Corn was the name for all cereal grains in old english [0]

[0] https://en.wiktionary.org/wiki/corn

[SECProto]

You don't even need to go back to old english: per your source, definition 1 under english etymology 1 is:

(chiefly Britain, uncountable) Any cereal plant grown for its grain, or the grain thereof.

edit: but yes, in the context of discussing a poem from the 12th century, old English would make more sense :) Interesting that the translation is from the french ble , which I would intuitively associate with the modern french blé meaning[1] wheat

[1] https://en.wiktionary.org/wiki/bl%C3%A9