Alliteration is used throughout "Farmhand," mostly to bring the poem's images to life. These images are used to highlight both the farmhand's awkwardness at the dance and his skill and strength when it comes to agricultural work.
In line 6, alliteration comes in pairs:
To the dance floor and the girls drifting like flowers
The two alliterative sounds—/d/ and /f/—suggest both the strong, punchy rhythms of the music in the hall and the delicateness of the dancing girls. The soft /f/ sound makes them seem gentle and almost otherworldly, while the /d/ projects youthful confidence (which the farmhand longs to possess).
In the third stanza, the poem focuses on the farmhand's physical appearance. He is described in line 9 as having "hairy hands," an alliterative phrase that is deliberately cartoonish and comic. This helps paint a kind of "Beauty and the Beast" scenario, with the farmhand seemingly lacking the physical traits that would make him feel more at ease at the dance.
The fourth stanza is full of alliteration. In lines 13-15, the poem constructs a kind of fantasy on the farmhand's behalf, as the speaker imagines what the farmhand desires (and lacks):
He has no girl to run her fingers through
His sandy hair, and giggle at his side
When Sunday couples walk. [...]
Notice how these alliterative sounds have a playful, carefree quality. The /h/ evokes the sound of laughter, while the /s/ sounds are whispery and intimate. In other words, the sounds help build an image of the girlfriend that the farmhand would like to have—and thus help underline that no such person currently exists.
In the last stanza, the poem describes the farmhand in his element. He "fork[s] stooks, effortless and strong," the clear alliteration here (as well as the consonance and assonance of this phrase) suggesting physical strength and skill. In the following line, the sounds are softer: the gentle /l/ sounds of "listening like a lover" reflects the sense of harmony that exists between the farmhand and one of his most important tools—the tractor.