non-glutinous rice
稉 centers on a specific type of rice: non-glutinous rice, the ordinary rice that does not become sticky when cooked.
稉 combines the grain radical 禾 with 更, which likely contributes a phonetic element. The exact historical development of the right-side component is uncertain.
The grain radical 禾 on the left shows this is about rice. The right side 更 can be thought of as 'again' or 'further,' so imagine ordinary non-glutinous rice being the kind you eat again and again, unlike sticky rice used only for special occasions.
For コウ, picture a bowl of non-glutinous rice being stirred with a big spoon that makes a hollow 'koh' sound: koh -> コウ, and the sound echoes as you prepare the everyday rice.