The other thing is local carrying capacity. A small herd might not have anything done to the manure, and through the wonders of bacteria and fungi not harm local streams at all. Once the operation is big enough to more closely resemble a Kansas feedlot, that shit's toxic waste.
And if the economics of megaproduction involve externalizing costs of dealing with waste, then somewhere we've broken something about pricing and/or regulation.
I'm not a farmer, I wonder if this idea that us govt price supports for farming leads us to have constant oversupply? And then what if they went away?
City dwellers can do nearly the same thing through farmers markets and Community Supported Agriculture services. (except instead of picking yourself, the farmer does the picking)
Mega-agribusiness is one of the consequences of increasing urban living. To support really large, dense cities, everything has to be scaled way up
Isn't it more a factor of how many people there are to feed rather than density?
A million people in a dense city will need (roughly) the same amount of food as a million people spread over half a state.
And that's assuming that each of these people are willing to work a half acre backyard farm, which is a lot of land to maintain as a hobby.
So they are consuming 400,000 acres (625 sq miles) of land, with half of it replacing dedicated agriculture, but the other ~300 square miles is going to low density housing. (which is impossible to serve effectively with transit so everyone has to drive to go anywhere)
In contrast, San Francisco houses 800,000 people on 49 sq miles of land. (which doesn't include the roughly 250,000 workers that commute there every day)
Mega-agribusiness is one of the consequences of increasing urban living. To support really large, dense cities, everything has to be scaled way up. A dairy farm is no longer producing for the 100,000 people in a 20 mile radius, it's producing for millions, and the waste it creates scales up too.
In an ideal world, these mega-producers would be less harmful to the environment than the equivalent number of small producers, because there would be some benefit from economies of scale. But in this world, the mega-producers largely see regulation and environmental caretaking to be cost centers and they pursue the cheapest solutions, which usually involve hiding the problem or paying somebody off or buying themselves some legislation. Smaller producers meanwhile have a harder time getting away with that.