Which sort of makes sense since the US has always had a huge agricultural / grain surplus.

米国 is because of ateji, not agriculture. 米 is the second character of 亜米利加 – an old transliteration of “a-me-ri-ka” as kanji. 亜 is the shorthand for Asia (亜細亜); the second character 米 is used as the shorthand for America. 米 is both the country (USA) and the continents – e.g. 北米 and 南米 are sometimes used for North and South America, respectively, while 米軍 is the US military.

Katakana has mostly replaced kanji transliteration of foreign words in modern Japanese, but some uses like the 米 shorthand persist.