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.