Cornish X are far from just a simple Cornish x Rock. Ridgerunner posted this on another thread the other day, and describes the process very well:
Back in the mid 1900's companies developed the first Cornish X meat birds by selective breeding. Two of the parent breeds they used were Cornish and White Rocks, hence the name of Cornish Rock that is sometimes used. It was never a simple cross of Cornish and White Rocks, it took many chicken generations to develop what they wanted. That was a bird that was very efficient on converting feed to meat, had a certain proportion of white meat to dark, and could get to butcher age pretty quickly. The younger they could butcher them the more flocks they could raise each year. In the 1950's this Cornish X pretty much replaced the Delaware, New Hampshire, and certain strains of White Rock as the preferred meat birds. They were just too efficient.
Since then they have continued to improve that bird. They are not so much trying to get it to grow faster, they could do that, but they are on a fine edge of also keeping them alive until butcher. It is not just genetics but how to maximize feed efficiency and how to house and manage them. They want a relatively robust chick that doesn't just die on them.
Several different companies have developed their own strain of these meat birds. They have invested millions into developing them and aren't ready to give that technology away for free. That is a highly guarded industrial secret as you would expect.
The basic model they have developed is that they have four grandparent flocks. Each of these flocks produce one of the grandparents of the birds that will mate to produce the broilers. One of these flocks will produce the grandmother of the father of the birds that will lay the eggs fro the broilers. A separate grandparent flock will produce the mothers of the mothers of the target laying flock.
Then they keep two more flocks. These are chickens hatched by crossing the appropriate grandparent flocks. Then they take the appropriate males and females from these two flocks to make the flock that actually lays the eggs that hatch into the broilers. It's not four flocks, it's seven.
These flocks do not need to be on the same premises, eggs can be shipped to a hatchery and baby chicks can be shipped to whoever is raising that flock. It's not that unusual for farmer to contract to raise one of these parent or grandparent flocks, but there are penalties to keep the technology secret. Also, these flocks have the genetics that make them grow fast. The people raising them have to manage them and feed them in a way that they don't get so big that they can't mate, let alone fall over dead. They have to balance how much to feed them to keep them healthy and laying eggs without overfeeding them to the point they die. An example of how they are refining genetics to help with the entire process, they have learned to use dwarfism in some of these flocks to stop them from growing so fast yet keep the dwarfism out of the final product.
