seed grain vs seed stock

seed grain and seed stock both are nouns.

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

As nouns, seed stock is a hypernym of seed grain; that is, seed stock is a word with a broader meaning than seed grain:

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

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