Welsummers for meat birds?
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You're not going to get a 5 pound carcass from a Welsummer hen in a reasonable timeframe for meat production. If you're talking about butchering two year old hens as stewing birds then you might get close if you have fairly large stock to begin with. You're also going to find that nearly every heritage breed of chicken has much more leg and thigh than breast. Large breasts are really a recent development with the cornish cross and the other 4-way hybrids.
I'm actually starting with Welsummers this year as an old school dual purpose project. I'm going to follow the Livestock Breeds Conservancy's guidelines for improving the breed with the hopes of getting close to their success with their Buckeyes. Having raised heritage breeds for meat before I am going to be quite happy with a 3.5 pound carcass at 16 weeks on cockerels and whatever my hens end up dressing out to when they find their way to the cone. I'm also planning to caponize the cockerels that don't make the cut at the 8 week assessment. I'm sure I have a few years of selectively breeding to come up with what I'm looking for out of the hatchery stock that I'm starting with.
I also really, really like heritage chicken meat. Since I had it for the first time I haven't had any illusions that it's a replacement or even a close analog to grocery store chicken. It's not even close. If you've never eaten a heritage chicken then your first step in research should be eating one of your birds. Read up on cooking methods. Just dropping a leg and thigh from an old hen into a sautee pan is the quickest way to cramp up your jaw.
If you want to raise a heritage breed for meat then you can go one of two ways. You can get hatchery grade stock; raise it, butcher it, and accept the outcome. Or, you can intensively breed for the traits that you want and develop a chicken that reaches a better slaughter weight at a younger age. I had a hard time finding someone that had done the work already that had chicks available for a price that I could afford.