Ted Hughes is considered a modern poet, so it's not surprising to know that he wrote this in a relatively modern format. "Full Moon and Little Frieda" is written in free verse. Free verse is a poetic form with virtually no rules. It doesn't have to use rhyme, and in fact free verse poems rarely do. There are no set line lengths or rhythms, either. Instead, poem written in free verse form is generally more about the varied cadence of the words matching the cadence or flow of the ideas. In this poem, the following lines help demonstrate the cadence of sound and meaning:
Cows are going home in the lane there, looping the hedges with their warm
wreaths of breath -
The "Cows" line is a long, meandering kind of line, indicative of the cows slowly wandering their way home; the "wreaths" line is short, like a puff of warm breath expelled on a cold day. The free verse form allows such diversity in lines and rhythms.