This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 米 so here's a mnemonic to help you:
The biggest problem when you buy unpolished rice is the mice (まい)! Mice like to eat and sleep in unpolished rice, so you have to be careful when you buy a bag that you get rid of all the sleeping mice before you try to cook it.
Unpolished rice is good for your health.