Say you have one rooster and six hens as in ridgerunners arrangement and you eat 45 chickens a year.
I mentioned 45 because that's what I eat each year.I'm not trying to make chickens a major part of my diet. There are only two of us also, not the four people in your example. If you follow my calculations using my assumptions you can get 210 chickens a year from 1 rooster and 2 hens. Since you are talking grams of meat and half of the chickens in my assumptions would be smaller pullets, get a third hen. That way you could raise 315 chicken year, plenty of grams. So that is four chickens, not 500.
My view, the claims about providing for the family and not supporting the meat industry are self righteous delusional nonsense....unless of course you have 500 chickens.
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Assuming the above figures are reasonable then the claims that backyard chicken keeping has any impact on the large commercial production of eggs and meat looks unrealistic to put it politely.
I don't see the vast number of people on here keeping chickens to try to have an impact on the commercial chicken industry, either eggs or meat. I see many people that would prefer to raise their own, even if the cost is greater, which it usually is. Some people will rant and rave about unrealistic things, but you'll get that in any large group. You can't assume that the fringes are the norm. I guess I really don't see where you are trying to go with this.
If you use your conditions as the only way you could raise that many chickens for the table, you are right, it would take a lot of chickens. But if my goal were to get half the meat I eat from chickens I raised, I would not use your model.
You could possibly adapt @Ridgerunner s model in a back yard but what you would be doing is running a small hatchery, not really keeping chickens.
I would want a lot more than 4 grams of protein a day to run a double incubator system, pullet pens (remember it's a back yard) and have hopefully a free range flock of the initial rooster and six hens.
So you consider keeping a rooster and 6 to 8 hens year around (and raising your own replacements) and hatching and raising 45 chicks a year for the table (some incubators and brooders, some broody hens) is not "keeping chickens"? Interesting thought process.
A free ranging flock or one rooster and three hens can provide enough eggs to hatch 300 chickens a year for a double incubator system if you collect the eggs. No pullet pens involved.
I'm not sure what your definition of free range is. I've had brooder raised chicks free range from age five weeks. No fences whatsoever. Here in the States that would meet the definition of free range. But after a few years I started having significant predator losses so now I use an area inside electric netting. Technically that would be pastured since they have forage year around. But free range is a different topic from how many chickens you would need to hatch 208 or 300 chickens a year.