seed corn vs seed grain

seed corn and seed grain both are nouns.

Word	Noun	Adjective	Verb	Adverb
seed corn	Yes	No	No	No
seed grain	Yes	No	No	No

As nouns, seed corn and seed grain are synonyms defined as:

seed corn and seed grain: good quality seeds (as kernels of corn) that are reserved for planting

seed corn (noun)	seed grain (noun)
good quality seeds (as kernels of corn) that are reserved for planting	good quality seeds (as kernels of corn) that are reserved for planting