I spent 6 years working on dual purpose Marans, after trying 4 different lines I finally landed on some Black Silvers that fit the bill better, needing less breeding work in the table side of things. I was on a mission to improve growth rate, fleshing, SOP traits and keep egg color good. Got pretty far into it before ending up with meaty but poor colored birds and well colored birds without the meat. Then I got distracted by another breed.
This is a whole batch of 18 week old Marans cockerels, they averaged 4lbs.
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What helped A LOT was selecting through 5-7 week old chicks for earlier expressions of width and weight. Then later on, after 16 weeks, selecting for length and depth.
These are 16 weeks old, Marans on the left and Bresse on the right. Same exact weight, just different shapes. The Marans had more length. This was at 5 years of breeding effort in the Marans, to get that fleshing all the way down the keel.
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Sorting birds by feel is very, VERY important! What they weigh could be a lot of bone. It's the feel of the fleshing and where the meat is that is much more important. If you look at the first picture, there's some lean ones down the keel length. That's what landed them in the shrink bag, for not passing the feel test.
This bird stay small and compact, with a short back. Then a bad attitude developed. So he offered a good view on what the carcass looks like when you get them widely built but lacking in length.
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With 3 to 5 generations of hard breeding selection and ruthless culling, you can achieve a LOT of improvements or changes.
This is where the egg color got to when I decided to move on to something else... Every season I'd hatch/grow about 150, to keep 15-20 for the following season.
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