One innovation changed pig farming drastically

Pigs and chickens have much in common: they eat similar diets (chickens, like pigs, are omnivorous) and grow to slaughter weight quickly - in less than two months for chickens and less than six months for pigs.

Unlike cattle, which require many leisurely months on pasture, chickens and pigs can be stuffed with feed and turned into meat in short order.

Unsurprisingly, then, the agricultural methods developed for one also worked well for the other.

In the 1950s and 1960s, American farmers started raising hogs like chickens.

Feed, augered into the hog barns from nearby silos, was deposited in automatic feeders.

Heaters and fans controlled the temperature, eliminating the need to open and close windows or haul straw for bedding.

The most important innovation was low-tech: slatted floors.

Used first in Norway in 1951 and adopted in the United States a decade later, the floors had long, narrow gaps that allowed urine and manure to fall into gutters below, where it could be flushed out with water.

"The use of slotted floors has probably accelerated the trend toward confinement more than any other single development," an expert wrote in 1972.

Slatted floors started a cascade of other changes in pig husbandry. Straw bedding, formerly needed to absorb urine and provide warmth, could be eliminated in favor of bare floors. There was no need for a separate dunging area, so more pigs could be packed into pens where they slept, ate, and relieved themselves.

For each pig weighing 150 to 250 pounds, industry guidelines in the 1980s called for allotting eight square feet of pen space, a dramatic reduction for animals that had historically been given free range of the woods or, at least, a pasture or sty.

In such close quarters, pigs kept each other warm, requiring less artificial heat, and gained weight more quickly because they didn't burn calories exercising.

Crowded together, they shuffled around more, trampling manure through the slots and keeping the pen cleaner.

Slatted floors made the farmer's life easier, eliminating what one industry publication called the "tedious and disagreeable" task of scraping manure from stalls.

Once a solid that needed to be shoveled, manure became a liquid that could be sluiced away.

The "comfort and convenience" of the farmer, an industrial manual reported, "may well be the most important" reason to move hogs into confinement.

The comfort and convenience of the pigs was left unmentioned.

Excerpted from "Lesser Beasts: A Snout-to-Tail History of the Humble Pig" by Mark Essig. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2015.