What aspects of capitalism do you think the stockyard and the animals symbolize in The Jungle?

The stockyard and the animals in it symbolize the dehumanization that Sinclair saw as intrinsic to capitalism.  

When Jurgis comes to America, he marvels at what he sees in the meat producing factories in Packingtown.  He is struck by the the rationality and precision with which it operated.  Sinclair describes it as "porkmaking by machinery, porkmaking by applied mathematics."  The stockyards and animals were conceived out of rationality and a sense of purpose.

However,...

The stockyard and the animals in it symbolize the dehumanization that Sinclair saw as intrinsic to capitalism.  

When Jurgis comes to America, he marvels at what he sees in the meat producing factories in Packingtown.  He is struck by the the rationality and precision with which it operated.  Sinclair describes it as "porkmaking by machinery, porkmaking by applied mathematics."  The stockyards and animals were conceived out of rationality and a sense of purpose.

However, over time, Jurgis recognizes that the plight of the humans and the animals are similar.  Sinclair writes that "... somehow the most matter-of-fact person could not help thinking of the hogs; they were so innocent, they came so very trustingly; and they were so very human in their protests—and so perfectly within their rights!"  In emphasizing the humanity in the animals, Sinclair asserts their link to workers.  Human beings were seen as essential to the capitalist scheme of the packers.  They entered "so innocent and so perfectly within their rights," yet were reduced to being cogs in a machine. Sinclair further describes how the animals were ground up "in this cold-blooded, impersonal way, without a pretense of apology, without the homage of a tear. Now and then a visitor wept, to be sure; but this slaughtering machine ran on, visitors or no visitors. It was like some horrible crime committed in a dungeon, all unseen and unheeded, buried out of sight and of memory."  The violation of the animals' rights is very similar to how workers were treated in the desire to make money.  Their rights as workers and as human beings were violated "without a pretense of apology." In the way that the animals were used as a means to an end, they symbolize the workers who toiled in Packingtown.

The stockyards were the backdrop for misdeeds to both animals and humans. Jurgis recognizes this when he sees how his labor is not valued.  The working conditions to which he is subjected and unfair compensation he receives represents the essence of inequality:  

All this was bad; and yet it was not the worst. For after all the hard work a man did, he was paid for only part of it. Jurgis had once been among those who scoffed at the idea of these huge concerns cheating; and so now he could appreciate the bitter irony of the fact that it was precisely their size which enabled them to do it with impunity.

The stockyard symbolizes where the worst in human interactions take place. It is where a person works and experiences mistreatment as a result.  Sinclair creates the stockyard as the setting where capitalism strips animals and humans of their dignity