This is a very short poem, just two sentences, and it packs a lot into them. The first, "Cows parse their dying," is odd and striking. To parse is to break something down and study it closely, as you would a sentence. So the cows seem to be working through their own deaths, almost thoughtfully, as if they understand what is happening to them.
The second sentence turns to people. "Vengeance" is a heavy word, and the poem suggests that the cows' deaths call up something like a payback, caused by "man's un-evolved appetites." The hyphen in "un-evolved" makes us stop and notice it. The speaker seems to be saying that humans, for all their claims to progress, still act on primitive hunger and greed.
The mood is cold, accusing, and unsettled. The speaker doesn't shout. The flat, clipped phrasing makes the judgment feel quietly severe. It's hard to say exactly whose vengeance is meant, and that ambiguity adds to the unease.