Here's why:
* Shafts of wheat isn't a common phrase. We usually talk about wheat in terms of stalks, heads, or fields.
* Collective nouns are typically used for groups of animals (a flock of birds, a herd of cows) or objects (a bunch of grapes, a fleet of ships).
If you're looking for a way to describe a group of wheat stalks, you could use:
* A stand of wheat
* A field of wheat
* A crop of wheat
Let me know if you have any other word puzzles!