This is the opinion of one speaker of BrE:
1. British English: plants such as wheat, barley, and oats or their seeds.
2. American English a tall plant with large yellow seeds that grow together on a cob (=long hard part), which is cooked and eaten as a vegetable or fed to animals [= maize British English]
Many speakers of BrE use the word in the American sense, though usually in such expressions as sweetcorn and corn on the cob.
1. the seeds of crops such as corn, wheat, or rice that are gathered for use as food, or these crops themselves
2. a single seed of corn, wheat etc. We use it this way in BrE, too
Does this mean "grain" does not include wheat, barley and oats? No, it doesn't. They come under 'etc'.
Jiang, if you are quoting from a dictionary, please name your source.
Student or Learner