I was reading that if you use the sprial mating plan with 4 pens it would be about 16 years before new blood would be needed, and 12 years for 3 pens and if you had only 2 pens then 5 years. I am wondering about 2 pens, but only replacing the family ( pen) in thier second year. Which means one year you replace one family and the next the other family ect. According to my calculations, if the above information is true, then breeding one family at a time should double the above numbers and mean that I would be able to go about 10 years 'till new blood. Does that sound right or is there something wrong with my thinking on that? I am thinking about using it with my silkies which are "brooders Only" birds with two families of 12 f and 2 males. One family sets on thier own eggs and the pullet family that is not being replaced could be used for hatching other eggs. What do you think?