panicled millet
穄 refers specifically to panicled millet, a type of grain plant. The meaning is narrow and concrete, centered on this particular crop.
穄 combines 禾 (grain plant) with 祭, which likely contributes the sound. The character was created to name a specific type of millet.
The grain radical 禾 on the left marks this as a plant. Picture a harvest festival (祭) where panicled millet is offered, tying the right side to the meaning.
For セイ, imagine saying 'say' as you point to a field of millet: 'say' -> セイ, and the grain is ready for the festival.