Siegel horse poem

Amazing. I love this guy (Robert Siegel) and hopefully will get some more of his books!
This poem is about a plow horse, sharp and strong imagery going on. One thing I didn't figure out is why he breaks some of his lines, and more often his stanzas, where he does... What does this shape do for this poem? I like to think something, but haven't discovered the plot yet.

From The Waters Under the Earth.

Versus
Versus... came to mean the turning of the plough, hence, furrow, and ultimately row or line. -Robt Wallace

We hear his heavy kick against the stall.
"No rest for the wicked," the farmer smiles,

shoving back the door. The dark inside
teases the nose with chaff. It takes them both,

father and son, to back him out, resisting
bit and blinkers, showing the white of his eye.

Harness and plow attached, he stamps,
sweeping away the retinue of flies,

nods and strains forward at the farmer's grunt -
head sideways, feet rising and falling like pistons.

The harness jingles, the plowman arches back,
rising the stilts as the coulter slices sod,

casting it in bright heaps. The plowhorse blows
flies from his lip, small stones click on steel,

black sod turns over. His neck muscles coil,
slide, and draw his head in tight

to a flared nostril, marble eye,
jaw wrenched and foaming. Meanwhile the son

dwindles behind the massive haunches,
jerking from side to side down the shining furrow,

until, tiny in the distance, the blade flashes
as he turns and starts a new row coming back.

Later that night at my desk, I still
breathe the rich humus on the damp air,

see that furrow stretch before me, moist ditch
rank with promise, crooked line

starting here, returning here, forever.