I received Dr. Carefoot's book, Creative Poultry Breeding in the mail. He writes that the slow feathering gene also causes slower development, but does not affect adult feathering. He states the very slow feathering gene retards comb development. He also states he was able to remove the slow feathering gene from his Barred Rocks without affecting barring.
Just another tidbit.
Angela
There is a link to slower development. All of the slow feathering breeds, historically, was slow to develop and mature.
The NHs stole the show because they were fast to feather, and fast to mature. More along the lines of the Mediterranean fowl that were more bankiva game fowl developed for egg production than any other thing.
Many of the large dual purpose breeds pre 1930 had the influence of Oriental breeds, directly or indirectly, and were slower to feather. Not all, and not exclusively, of course. There was different feather types then to.
It should also be noted that Barred Rocks in their prime, were not as slow to develop as they are now. Thompson spoke of birds that were finished (ready to show) by 24-28 wks. Broilers (not as we know them) @ 6-8wks etc. For a time, they were at a peak in beauty and performance. They were not as painfully slow to develop sexually as they are now. Since they have digressed to the performance of their founders.
The book and author that you speak of is English. Their view on Barred Rocks, and their barring is different than ours. The barring on their birds is clean however. Still different. Honestly, I prefer what they did with this characteristic, over what we did do. The Delaware projects neglected to realize that slow feathering is not necessary for clean barring. I think that is a case where clean, crisp barring though wider is a benefit. We have been taught that crisp barring is a result of slow feathering. Not necessarily, if we are willing to accept wider barring and wider feathers (which I think is better).
A friend of mine that was working on a more productive line of Barred Rocks, and I, have discussed this point along the way. Her efforts did show that you can get reasonably good developing birds, productive birds, and clean barring in one bird. Maybe not enough to win the Nationals, but far and away much better than you could ever dream of getting from a hatchery.
There are some cases where we can fatally wound a breed over the obsession of a single characteristic.
My opinions only.