However, you could use these options, depending on the context:
* A heap of rice: This is a good option for a large quantity of rice.
* A bowl of rice: This is suitable for a portion of rice.
* A field of rice: This is used for a large cultivated area of rice plants.
* A crop of rice: This describes a specific harvest of rice.
Ultimately, the best collective noun for rice will depend on the specific situation.