Studies by the broiler industry involving NN introduced into broiler stock seem to say NN birds have more meat yield than their non NN relatives and control outgroups. The difference is more apparent in hot climates- a lot of the studies I came across were for broilers in hot desert areas where there was either high losses or special measures having to be taken for their survival and the study was to check if NN gave any advantage.
The NN continued to eat because they were able to stay cooler(they took internal temperatures) and less stressed than the non NN so they were able to gain more weight.
The difference was not so great in cool/cold-ish set up however if I understood them right, even while the NN did not grow faster/bigger, they still had more meat yield per live weight. Seemed like the NN gene had a side effect of making more meat on the birds. Don't hold me to this.. those are not exactly the easiest reads and my eyes get crossed reading them for too long... the stilted language does not help much.
I have to remember all these studies were done with broiler type stock so those NN are not like 'our' NN though.. I see a lot of variation in my birds but it is due to a lot of mixing and separate lines with different selection... it does seem the NN ones are generally heavier feeling than the non NN but that's without being able to give comparable weights on them though.
I definitely see a huge benefit to the NN as the summertimes get to 110-120F, which kills the heavier or meat bred breeds here.. I tested for it by introducing NN into unfortunately proven heat-sensitive breed and there was a big improvement in heat tolerance.
Besides the broiler stock studies, free range NN are very popular table chicken in France along the Bresse.
Thanks Kev, so if I add NN to my project which is aiming rather loosely lately as they are a mixed up bunch, it may add some more heat resistant to them. They already survive our summers here which rarely go over 110*
I'm ok with their size and the meat is very good. Eggs are starting to pick up now with Spring trying to get here and they are about 10 months old. I was hoping they'd do good in winter with egg laying. With their age and trying to get started in the winter it really wouldn't be a fair judge yet I suppose. Does everyone get their start from hatchery birds or is there a good source on BYC for eggs or chicks.