Tip 1:

Consider the graph where nodes are pastures and edges are for
cows. There are two types of edges depending on whether the cows eat
the same types of grass or not.



Tip 2:

The number of ways to plant pastures corresponds to the number of ways
to bicolor the graph. Is it always possible to bicolor the graph?  If
you know that the first pasture is planted with the first type of
grass, what does this imply on other pastures?


Tip 3:

There are 0 or 2 ways to color each connected component of the graph.
For each connected component you can set any node of that component to
a given color and propagate the constraint, you will either manage to
color the full component or end up with a contradiction (a pasture
that needs to be planted with two different types of grass).