I think one of the governing images in this poem is the mirror in line 4. The idea of a mirror is significant, as the entire purpose of the poem is to compare the little girl Frieda (Ted Hughes and Sylvia Plath's daughter) to the moon, as they look at each other in amazement.
Much of the poem describes the orderliness of the environment. The spider's web is "tense" and taut, and the cows (in the lines you asked about) are perfectly ordered as they move along the road, all breathing in identical patterns. They are so orderly that even the milk in their udders is "balanced." The cows' "warm wreaths of breath" gives an image of how the cows' warm breaths are visible in the cold air.
However, interrupting all this stillness, Frieda notices the moon and "suddenly" cries out. Hughes likes to imagine that Frieda's outburst has caused even the moon to turn and look at her, and admire what a perfect work of art she is. In this respect, one might consider the poem to be an ode to Hughes's daughter (certainly in message, if not in form).