Open nucleus breeding systems have been shown to result in higher genetic gains and lower rates of inbreeding than closed systems. The organisation of an open nucleus system is, however, complex and the long term success depends upon a consistency of objectives among, and strong commitment from the participating breeders. Initial genetic differences between flocks ultimately result in a greater advantage of the open nucleus system, but the short term effect, for the better flocks, is a reduction in genetic gain compared to closed flock selection. The open nucleus system also involves the physical transfer of breeding stock between flocks, which may represent a risk to sheep health.

There are, at present, a number of sire referencing schemes operating in pedigree sheep populations. These schemes involve the mating of ‘reference’ sires to randomly nominated ewes in a number of flocks. The genetic links that are created between flocks enable accurate comparisons of estimated breeding values and therefore accurate selection across flocks.