Why had “the stench of the place been rising for more than a mile?” from the omnivore's dilemma

From the text:

And then, very suddenly, the golden hills turned jet-black on both sides of the highway: black with tens of thousands of cattle crowded onto a carpet of manure that stretched as far as the eye could see. I was driving through a feedlot, with tens of thousands of animals bellying up to a concrete trough that ran along the side of the highway for what seemed like miles.