The answer is : Feedlots are the key to the American system: one source says 30 million cattle annually are finished for slaughter in this way.
Those lagoons in the aerial photo are sewage. The big feedlots produce more effluent than many large towns, creating a significant environmental problem. The South Lot of the Monfort Beef operation in Greeley, Colorado, has been claimed to be the world's largest feedlot, holding 120,000 head integrated with a feed mill and a meat packing plant. Nebraska is the state claiming the most head and the most feedlots.
Well at least they are outside and not inside a huge shed all their lives...