The result was the factory farm, first for chickens, then pigs, and more recently cattle. Producers discovered that animals could be kept inside, and fed grain, and could be bred to grow more quickly and get fatter in the right places.
Since 1925, the average days to market for a US chicken has been reduced from 112 to 48, while its weight has ballooned from a market weight of 2.5 pounds to 6.2.
Pig and cattle farming has followed suit. Sows are held in gestation crates for up to four weeks once they are pregnant, and then put into farrowing crates once they’ve had their piglets to prevent them accidentally crushing their young. Industrially reared pigs spend their lives in indoor pens. Cattle farming is now being similarly streamlined, with cows in the last few months of their lives being fattened in feedlots with no access to grass and sometimes no shelter.