Last year I let all the flocks free range together, so I hatched a lot of mixed breeds. I had a Black JG and Blue Copper Marans roosters doing most of the "work". My hens were different colors of marans, a couple of easter eggers, black JG, coronation sussex, cochins, and an assortment of others. I tried to hatch just the marans and easter egger eggs and filled in a few blank spots with the biggest most perfect eggs from everyone else. I managed to finish another small coop and pen, so when they were grown out enough to go outdoors I separated them into pullet and cockerel pens. The pullets were big and beautiful; sold all of them for $15 a piece last fall and this spring to folks who were thrilled with them. This is 4-6 months old and out with Flopsy the legbar rooster.
The cockerels grew quickly and were at processing weight at between 6-8 months. I started out with 12 and am down to 6 without processing any yet because people who came to buy pullets were so taken with them that they kept buying them. Just saying that the JG and marans crosses make a lovely meat bird that has good size and grows moderately quickly. The ones I did last year were from 4-6 pounds. Doesn't sound like much, but the meat is so dense and rich you don't have to eat as much to feel full. One would give me 4-5 meals as a single person.
The JG cockerel I processed last year was a year old and huge. He was the scrawny one
JGs are not fully mature until 2 years old, but you can get a nice carcass at a year if you want to cull then. They grow up for the first year and out the second year. That's the beauty of the crosses. They retain some of that bulk while they grow faster. The marans/JG and sussex/marans or sussex/JG were the stockiest.
