Number of founders for a breeding population using variable parental contribution

This study considers a situation in which there is access to a high number of progeny tested plus trees, of which the best ranking ones are used as founders of the next generation breeding population. The question was how to obtain maximum genetic gain with a given level of genetic diversity. The level of genetic diversity was fixed to the level obtained with equally contributing within-family selection, and the resources were fixed by keeping the number and size of full sib families constant. The between-family genetic gain in establishing a breeding population was maximised by means of deterministic modelling. This was performed by selecting more founders than in the case with equal contribution and allowing them to be represented in the next generation by one, two or three offspring depending on their breeding value. The effect of variable contribution was studied by altering two parameters: selection intensity (number of tested plus trees) and genetic diversity in the resulting second generation breeding population (status number). An increase of 20 to 23% in the number of founders as compared to equal contribution gave the maximum genetic gain when the genetic diversity was kept constant. The optimum founder number increased with increasing number of tested plus trees and status number. The size of the highest contributing fraction of founders increased with increasing number of tested plus trees and decreasing status number. The amount of additional genetic gain obtained by variable contribution was greater with a smaller selection intensity and lower status number. In a typical case the between-family genetic gain could be increased by 7% by increasing to the number of founders by 22% and allowing them to contribute unequally to the next generation. Most of the additional genetic gain was obtained after the first few founders had been added