Breeding Cattle/Horses On Hills

Is there any reason I cannot bread Cattle/Horses on hills. Yet I can build a pasture on them?

The pasture is for use with sheep which is the only livestock that can thrive on hills.

This may need to be updated in the civlopeida. I will look up the part that confused me when I get home, but I seem to recall it mentioning that cattle, sheep, and horses could be on hills. Though I may have just misread it. May be a good idea to put a note in there that only sheep work on hills.

Thanks for the reply,
Bob