What is the collective noun for plains?

There isn't a widely accepted collective noun specifically for plains.

However, you could use descriptive phrases like:

* A expanse of plains

* A sweep of plains

* A stretch of plains

* A sea of plains

You could also use a collective noun that describes the animals that inhabit the plains, like:

* A herd of plains animals

* A flock of plains birds

Ultimately, the best option will depend on the context and the specific nuance you want to convey.