Abstract
Beef cattle breeding system was investigated theoretically by applying the nucleus breeding system where a sire breeding nucleus is either closed or partly open. The initial genetic superiority of the nucleus over the commercial is quickly passed to the commercial, indicating that the larger the initial superiority the quicker the genetic gain in the commercial immediately after the start of the selection program. After the steady state is reached, genetic gain is achieved only by selection differentials accumulated in the nucleus. When the nucleus is closed selection differentials for animals used in the commercial contributed only to reducing the improvement lag of the commercial from the nucleus. The extent to which the commercial is behind the nucleus is, whether the nucleus is closed or partly open, two times the mean generation interval in the commercial with selection differentials for animals used in the commercial reducing the lag.
