A DETERMINISTIC MODEL OF BLUP SELECTION IN 2 TIER OPEN NUCLEUS BREEDING SCHEMES

A deterministic model is proposed for selection response in two tier open nucleus breeding schemes in which selection is based on breeding value estimates (ebv) calculated using a BLUP animal model. The population structure which maximises the equilibrium rate of genetic gain is determined for various open nucleus schemes selecting on BLUP ebv, and accomodating variance loss due to selection. The percentage improvement over a closed nucleus in the equilibrium rate of genetic gain of an optimised open nucleus scheme is independent of selection accuracy and thus depends solely on selection intensity. Moreover, optimum nucleus size is also independent of selection accuracy. These properties do not hold for mass selection within tiers. In sheep and beef cattle breeding the maximum advantage from opening a closed nucleus is between 8 and 13% for BLUP selection allowing both male and female migration between tiers. For selection on the basis of regressed Within Tier Phenotypic Deviation (rWTPD), the maximum percentage improvement in the equilibrium genetic gain from opening the closed nucleus decreases as heritability increases and approaches a lower limit determined by BLUP selection.