In "I Knew a Woman," what do the seed, grass, and hay metaphors mean?

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."