I think that I did this, unintentionally, back in 2009. The Sand Hill Dorkings that I introduced to my flock at that time produced all kinds of colors. I'm hoping that I've finally got that sorted out. I won't cross varieties again.
I have, over the years, combined SGDs from at least five different sources. Yet, the hatchability issues remain. So fresh blood did not help. I am looking into getting one particular line, that I'm hoping will help.
I thought that diet may be the cause. I've tried different feeds. I'm feeding a breeder ration this year and hatching two breeds. The hatch rate on the Delawares is 3 times higher than the Dorkings. So I rule out diet as the problem, or else the Delawares would also be suffering. Unless the Dorkings have some dietary need unique to the breed.
I started pedigree mating last year and keeping detailed records. I only kept the very best producing hens and the offspring from them that were closest to type. I expected better results this year, but am having both fertility and hatching problems. I am still seeing strengths in individual hens.
The chicks that hatch are very vigorous. I don't use medicated feed or immunize or baby them. If they manage to hatch, they do great.
Hoping that the strong culling & record keeping will eventually result in better hatchability.
I always hate to hear such negative banter about the Silvers. There are some breeders who have been working with them and one in particular has been doing quite well in shows this past year. For the person who wants Reds, get Reds if that's what you desire. There are nice ones out there.