Translation:Rice is the main crop of that region.
No, there is no liaison between the subject and the verb. http://www.lepointdufle.net/ressources_fle/liaisons_obligatoires_liaisons_interdites.htm
Cultivar mean something else: it refers to a cultivated variant of a crop, not the crop itself. The Granny Smith is an apple cultivar, but apples themselves are not a cultivar.
If you were referring to, for instance, basmati rice specifically, you'd be correct. However, simply accepting cultivar as equivalent to crop is not correct, so I wouldn't support adding it as a valid alternative.