Here are some options, depending on the context:
* A field of grain is a common way to describe a large amount of grain growing together.
* A crop of grain refers to the harvested grain from a specific planting season.
* A sheaf of grain is a bundle of grain stalks tied together, often used for harvesting.
* A bushel of grain is a unit of measurement for grain, typically used for storage and trade.
Ultimately, the best collective noun to use will depend on the specific situation and what you are trying to convey.