Feed lots are very highly concentrated populations of cattle that are fed a regulated diet of grain (corn) mixed with concentrated antibiotics and, in this case, rotten chocolate. Another commenter already referenced Pollan's work, which exposes the tradeoffs of this industrialized approach. Among these are increased e.coli outbreaks (as e.coli concentrate in the rumen of the cow when it tries to digest grain,) and the increased likelihood of antibiotic-resistent bacteria developing because of the tremendous amount of antibiotics used to keep animals living in these conditions from getting sick:
http://www.oklahomafarmreport.com/wire/news/2010/07/media/00...
(That ankle-deep black stuff is poo, which also runs and collects in giant sewer pools that generate toxic runoff.)
Since grain (or in this case candy) has a much higher energy content than grass, cattle can be raised to market weight much faster than if they were fed a grass diet.
Edit: I accidentally a word.