United States
America, USA
proper noun, の adjective
Rice country is the United States, otherwise known as America. Maybe they call it that because of all the rolling plains in the middle of the country? You could grow a lot of rice out there.
米国 is a more formal written term for the USA. You'll see it in news articles and official documents, but in everyday speech people usually say アメリカ.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
主人は米国にいます。
My husband is in the United States.
米国へ入るには、パスポートとビザがいります。
To enter the United States, you need a passport and visa.
米国は五十の州が有ります。
The United States has fifty states.