I got an answer! It's white rice.
But the historian who answered me cited two works:
Bray, Francesca. Science and Civilisation in China Volume 6, Part 2: Agriculture. Cambridge: Cambridge University Press, 1984. 381-86.The first book is one I knew existed but costs something like $300 on Amazon. So its contents are a mystery. But the second one is the same book I've been working on translating... and there's apparently an English translation already floating around.
Shi, Sheng-han. A Preliminary Survey of the Book Ch’i Min Yao Shu: An Agricultural Encyclopedia of the 6th Century.Peking: Science, 1974. 53-54 (an annotated English-language translation of Qimin Yao Shu).
In my defense, I didn't find it because that translation is using an older romanization scheme than I'm using, so searching for "Qimin Yaoshu" doesn't turn up "Ch'i Min Yao Shu." But regardless, I'm both excited that I can access the information more easily and disappointed that my work isn't as novel as I had hoped.
The lovely person who responded to my question on reddit also gave me a picture of the bibliography tables from Science and Civilization in China:
... but also I've found a digital copy of the entire book floating around the web by googling "Science and Civilisation in China Volume 6 Part 5," so I'm working on reading it to get more info out.
I think my path forward is this: buy the English copy of Qimin Yaoshu, but only read the chapters I'm not translating, and when I'm done with translating what I'm working I'll cross-check and see what the differences are. I'm not sure I'm going to trust Shi Shenghan's translation without cross-checking.
And of course, I want to take a crack at the rest of the books in this bibliography. They're closer to the typical medieval period, and I suppose they're less likely to be already translated.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.