Eugenics
Explanation
The Joke
A character argues that eugenics is bad but that humans have been doing selective breeding with cattle for hundreds of years, making them "better and better" — so in that sense, cattle are the "superior race." The rebuttal comes when another character points out that selective breeding in animals doesn't mean what eugenics proponents think it means: many generations later, the comic shows the descendant cattle looking back at their ancestors and saying "your new self-immolating flank is ready," implying that the cattle have been selectively bred to the point where their flesh self-cooks for human convenience. The "superior" bred organisms were optimized not for their own benefit, but for easier exploitation by their breeders.
The comic takes the pro-eugenics argument — "look how well selective breeding works on animals" — and follows it to its darkly logical conclusion. The cattle weren't bred to be "superior" in any meaningful sense for themselves; they were bred to be more useful to the humans who eat them. This undercuts the idea that selective breeding produces genuine superiority, exposing the uncomfortable truth that "better" in a breeding context means "better for the breeder, not the bred."
The Humor
The dark comedy comes from the final image of cattle cheerfully announcing that their self-immolating flanks are ready — breeding taken to its absurd extreme, where the animals have been engineered to literally cook themselves. It's a reductio ad absurdum that simultaneously satirizes eugenics arguments and highlights the ethically troubling nature of industrial animal breeding. The juxtaposition of the proud tone ("your new feature is ready!") with the horrifying implication (the animals exist solely for consumption) is quintessential SMBC dark humor.