@Nyboy you have a few limiting factors. As Bob mentioned you have genetics dictating rate of growth that define a breed, and calories/protein/fat also as a limiting factor.
If you are not able to meet the demand for calories, protein and fat that the birds rate of growth need you will have reduced growth rate. I've seen a few threads in the past with people who deliberately slowed the growth on cornish crosses in an effort to get them to breed.
They got the slower growth but I can't remember if they were able to reproduce.
For some reason pink seems a good color to highlight with this morning.
Jared, the Cornish X can reproduce if you limit the diet enough to keep them alive but you also include the right nutrients. It's a real science, feeding them the right mix of nutrients and the right amount of feed. The hens will lay a pretty good amount of eggs, though they are more medium than large. Their mothers and grandmothers are production birds whose job is to lay a lot of hatching eggs so laying a lot of eggs is a trait they breed into them. Size of the eggs isn't important as long as they are hatchable.
The hens can get pretty large anyway but a regular full-sized rooster can cover them and fertilize the eggs. The males will also get huge, if you don't know what you are doing with the feed they can get so big they can't breed. You might need to use artificial insemination with them.
The parents of the Cornish X would make great meat birds without crossing them, the genetics are there. They have to limit their feed so they are able to reproduce. These videos show how one producer does that and quite a bit more about breeding them.
@Nyboy I don't know what mix of produce you are talking about. It is possible you could keep Cornish X chicks alive and even grow them on garden produce if you get the right mix. You might consider it cost effective if you are feeding them garden scraps or excess. I would not try it for all the reasons they have mentioned. People do supplement chicken feed with what they call treats, like garden produce, even with Cornish X. But those treats are limited so their main feed is the bulk of what they eat. It is not the sole diet.
To grow they need vitamins, minerals, fiber, fats, protein and more. They need certain amino acids that come from animals and are usually added as supplements to chicken feed. We are talking about organics on another thread. Those amino acids are so badly needed that the organic rules allow artificial supplements to be added to chicken feed. Man-made, not natural. That's how important they are. Those are the funny words on the analysis on the label. What produce are you going to feed them that contains all this, especially proteins and minerals in the quantity they need?
Another issue is that the Cornish X are specialists. They have been bred to produce a lot of meat in a very short time. They are very good at converting what they eat into body growth. But they are not very robust. They are pretty delicate. They are not bred for longevity. Their body is fine tuned to produce a lot of meat on a pretty special diet. If you get too far away from that their bodies may not be able to handle it.
You can try, you might even have something you consider success. I don't know where this is coming from, it sounds like a nephew thing. But I think you are setting yourself up for real disappointment.
