General terms:
* Heads
* Spikes
* Shoots
* Stalks (when referring to the entire plant)
Specific terms:
* Wheat heads (for wheat)
* Corn ears (for corn)
* Rice panicles (for rice)
* Oat panicles (for oats)
* Barley heads (for barley)
The best word to use depends on the context and the specific grain you are referring to.