Jiugukao 九穀考 is a book on staple grain written by Cheng Yaotian 程瑤田 (1725-1814), courtesy name Yichou 易疇 from Shexian 歙縣, Anhui. Cheng wrote several books on issued related to farming, like Gouxu jiangli xiaoji 溝洫疆理小記, Shuitian xiaoji 水田小記, Shichong xiaoji 釋蟲小記 or Kaogong chuangwu xiaoji 考工創物小記.
The book with a length of 3 juan analyses nine types of "grains", namely broomcorn millet (shu 黍 Panicum miliaceum), non-glutinous millet (ji 稷), rice (dao 稻, Oryza sativa), foxtail millet (liang 粱, Setaria italica), sesame (ma 麻, Sesamum indicum), soybeans (dadou 大豆, Glycine max), azuki beans (xiaodou 小豆, Vigna angularis), wheat (mai 麥, Triticum spec.), and Manchurian wild rice (gu 苽, Zizania latifolia). The basic text for the analysis is the ancient character dictionary Shuowen jiezi 說文解字, while the statements found in this book are enriched by quotations from a rich host of other types of texts, from the Classics and history books to the "masters" and farmer's proverbs. All these texts provide detailed information on the whole process of cultivation, from the preparation of the soil to the harvest and the processing of the crops.
Cheng explains that liang 粱 was the designation of the grains of unhusked millet (su 粟), that there are a glutinous and a non-glutinous type of shu 黍, and that there was indeed a difference between ji 稷 and liang 粱. Yet is identification of ji as gaoliang 高粱 is a great error. An interesting aspect is that crops have different names during different periods of growth, for instance, the term fu 稃 "husk" for parts of the Manchurian wild rice. Concerning this plant, Cheng also describes some fungus diseases (ergot).
Apart from the nine types of grains, the book gives explanations to grain plants or mature ahead of time (zhi 稙), young grain (zhi 穉), sowing (zhong 種), fast-maturing grain (lu 稑), seeding (jia 稼), and harvesting (se 穡). The book ends with a treatise on the origin of ancient weight measures in fixed amounts of grains of unhusked millet (Zhu fa qi qu shu su 銖法起于黍粟).
Cheng's book inspired Liu Baonan 劉寶楠 (1791-1855) to write a similar book on grain, Shigu 釋穀, for which Liu quoted intensively from the Jiugukao.
The appendix consists of four texts consisting of notes and letters concerning detailed questions. The book was first printed in 1860 by the Xuehai Hall 學海堂 in Guangdong as part of the series Huang-Qing jingjie 皇清經解. In 1933, it was integrated into the series Anhui congshu 安徽叢書.