The poem's bold, colorful metaphors and similes bring the "Sow" and her surroundings to vivid life.
For example, the phrase "Maze of barns" (line 9) suggests that the sow lives on a rather large farm, one with a complex series of structures and outbuildings. The reference to a "parsley halo" (line 16)—the sprinkling of herbs that garnishes a cooked pig—casts slaughtered livestock as martyrs of a sort. The "Sow" of the poem has evidently avoided this fate; she has lived for herself rather than being sacrificed to others.
Later, the speaker compares the typical "barnyard sow[]" to a "Bloat tun of milk," meaning that adult female pigs are milky-white, bloated, and shaped like a barrel or tun. They find food by going on a metaphorical "snout-cruise," or sniffing along the ground. They are often "hedged," or surrounded, by piglets ("a litter of ninnies"). These offbeat metaphors have the effect of defamiliarizing a familiar animal, reminding the reader how comically awkward-looking pigs are.
Toward the end of the poem, metaphors and similes cast the main "Sow" as a sort of mythic creature. When she wakes from her dreams, the "legend" of past glory seems to fall off her like flakes of "mud." Sure, this simile means she's returning to her normal, humble life as a pig, but it also invests her with a lingering aura of the legendary. And as she rises from the mud, she looks anything but normal. The speaker compares her to "that hog"—some creature of myth or legend—who can't "stomach[]" (accept) any limits on its appetite, and "Proceed[s]" to "swill" the whole world. (In other words, devour the "seas" and "continent[s]" as if they were scraps of pig feed.) Both in her dreams and in reality, then, this sow is a force to be reckoned with.