In addition to representing the life cycle, the seed--grass--hay metaphor carries the connotation of fertility (sowing one's seed) and youth (green grass) and finally reaping the results of one's life/work/lovemaking. The hay could also represent the speaker at his most mature stage--no longer part of the fertilization/reproductive process, he is golden, awaiting the reaper, soon to return to the earth, leaving only the legacy of what he once was.
Let seed be grass, and grass turn into hay:
I'm martyr to a motion not my own;
What's freedom for? To know eternity.
I swear she cast a shadow white as stone.
But who would count eternity in days?
These old bones live to learn her wanton ways:
(I measure time by how a body sways.)
Roethke's poem is about a man who is madly in love with a woman. He is captivated and captured by her, and he is happy to be under her control. Seed, grass, and hay refer to the cycle of life. The speaker is saying that life goes on all around him, but he hardly notices it. He no longer measures time by the seasons or by the cycle of birth, life, and death. He measures time "by how a body sways."