seed stock vs seed corn

seed stock and seed corn both are nouns.

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

As nouns, seed corn is a hyponym of seed stock; that is, seed corn is a word with a more specific, narrower meaning than seed stock:

seed stock: a supply of seeds (or tubers) reserved for planting
seed corn: good quality seeds (as kernels of corn) that are reserved for planting

Other hyponyms of seed stock include seed grain.

seed stock (noun)	seed corn (noun)
a supply of seeds (or tubers) reserved for planting	good quality seeds (as kernels of corn) that are reserved for planting