References to "corn" in translations of the Jiuzhang Suanshu

One passage from "Nine chapters on the mathematical art" dealing with solving simultaneous linear equations is sometimes translated as follows: "There are three classes of corn, of which three bundles of the first class, two of the second and one of the third make 39 measures. [etc.]" But corn (as the term is used nowadays) is a New World crop, so what would be a better translation? ("Grain" seems safe, but perhaps something more specific was intended.)

Merriam-Webster:

corn 3. British : the grain of a cereal grass that is the primary crop of a region (such as wheat in Britain and oats in Scotland and Ireland)

Full disclosure I do not speak any Chinese of any variety. I am fully reliant on the sources.

The following sources contains translations of the passage:

Put down 3 bundles of top grade cereal, 2 bundles of medium grade cereal, 1 bundle of low grade cereal and 39 dou as shi in a column on the right.

Top-grade ears of rice three bundles, medium grade ears of rice two bundles, low-grade ears of rice one bundle, makes 39 dou.

Suppose we have 3 bundles of high-quality cereals, 2 bundles of medium-quality cereals and one box of poor-quality cereals, amounting to 39 dou of grain;

From the German literature:

Aus 3 Garben einer guten Ernte, 2 Garben einer mittelmäßigen Ernte 1 Garbe einer schlechten Ernte den Ertrag von 39 Tou.

"Ernte" simply means harvest, hence Vogel avoids the crop type completely.

I have as yet to identify a source for the "corn" variant of the translation though it is used on MacTutor without precise citation to a translation.

P.S. See Gerald Edgar's answer for a plausible explanation of the meaning of corn. This further suggests that the given translation might be of British English origins.

The original seems to be 今有上禾三秉，中禾二秉，下禾一秉，實三十九斗 (from here).

The character that got translated to "corn" is 禾. My dictionary (pleco) translates this character to "grain seeding (esp. rice)". I believe they really meant to write "seedling" instead of "seeding".