Recommended approach:
Spell the term "grain"
If needed for storytelling, after establishing strong context -- you can depict an individual grain with a classifier-G
After establishing context via fingerspelling, some signers use the traditional / older sign for CEREAL brushing the x handshape of the dominant hand along the forearm of the non-dominant hand representing a scythe reaping wheat. Generally uses a double movement but the movement may be reduced later in the same conversation.
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During storytelling if by grain you mean "seeds" you could use the sign for "seeds"
See: SEEDS
Notes: You may some people sign grain uses a "G" handshape on the dominant hand to show small bits of grain as if lined up on the index finger of the non-dominant hand. However this tends to conflict with the sign for peas.
Contrast with: PEAS
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