At midday, cattle wander up to a rose and eat it. That small, ordinary event is the whole scene, but the poem gives it a strange weight. The animals take the flower's head, and the poem treats this almost like a beheading. The final image is the oddest: the severed head still seems to speak "dark thoughts / to itself," as if the rose goes on brooding after it has been taken.
The comparison in the middle is the key. The cattle's arrival is "like a sudden rift / growing in a ruin," a crack opening in something already broken down. That suggests the rose was growing somewhere neglected, and that the loss is both quiet and final. The speaker seems to watch without comment, which makes the mood detached and a little eerie. The title's plain, scientific word, "Herbivory," sits against the violent language, and that gap creates the unease. Nature is just feeding, yet it feels like something grim and private is happening.