Data on the reduction in improving effect of A.I. Bulls in relation to the genetic trend of the population

In order to clarify the question of the reduction in improving effect of A.I.bulls in relation to the genetic trend of the population, the authors have carried out two model calculations: a) This one assumed different genetic progress as well as different improving effects of bulls. The duration and extent of improving effect have been examined. b) The data on an US-Holstein-Friesian elite population was processed to determine changes in improving effect during a period of 10 years, assuming 1 per cent genetic gain per year. Ten per cent of this elite population were chosen and examined as a 'nucleus population'.Conclusions are as follows: 1. In female populations representing continuous and considerable genetic progress, even bulls of excellent genetic merit can assure improvement for only a few years. 2. The improving effect may be referred to a definite population only if the progeny test results are directly related to the average of the population, or if these can be adjusted properly. 3. A 'nucleus population', selected and maintained by high selection pressure, could have such a great genetic superiority that progress is assured for a long time in the 'population with high breeding value'. 4. A significant genetic advance has to be achieved, maintained and utilized in the sire-producing mating category. This requires reasonable rationalization of selection and mating, utilizing international gene-resources and continuous control of the breeding value of sires in the populations.