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The present article investigated how the expected genetic gain of weaning weight changed in relation to the size of the nucleus and commercial flocks in order to develop a suitable strategy for improving meat production in Barki sheep. The original body weight data were collected from the Barki sheep flock of the Desert Research Centre from 1963 to 2005 with a total number of 1046 animals (542 females and 504 males) progenies of 163 sires and 557 dams. The simulated sets of data were generated from the original data and designed to construct two scenarios of an open nucleus breeding scheme each consists of a nucleus and 5 contributing commercial flocks. The first scenario concerned with small flock size (1500 ewes and 50 sires in the nucleus flock together with 3000 ewes and 100 sires in each commercial flock), while the second scenario dealt with large flock size (3000 ewes and 100 sires in the nucleus flock in addition to 6000 ewes and 200 sires in each commercial flock). The open nucleus breeding scheme was also simulated to allow for the exchange of superior sires among tires in both directions. Four generations of progenies were obtained by selection of sires and dams for the next generations. The annual genetic gain as a response to selection for weaning weight was also calculated. Results of breeding values of sires, dams and offspring as well as average weaning weight indicated no differences between large and small flock sizes despite the slight increase observed in the small flock size throughout four generations. Breeding values of sires, dams and offspring as well as the average weaning weight tended to increase as generations increased from first (G1) to the fourth generation (G4) in nucleus and commercial flocks in both small and large population size. Selection for weaning weight through applying open nucleus breeding scheme for four generations increased average weaning weight from 19.02 kg to 32.58 kg (by 71.3%) in the nucleus flock and from 16.33 kg to 28.91 kg (by 77.0%) in the commercial flocks of the small population size. The corresponding values for the large population size were 18.67 kg to 32.50 kg (by 74.1%) in the nucleus flock and from 16.30 kg to 28.56 kg (by 75.2%) in commercial flocks. The annual rate of genetic gain for average weaning weight tended to decrease as the generation advanced from G1 to G4 in both small and large population sizes. There is a considerable increase in the genetic response in both large and small population sizes despite the slight difference in the annual genetic gain of weaning weight occurred in small flock sizes. The study argued that without increasing the number of animals in the nucleus, breeders could accelerate genetic progress in productive traits by annually screening elite animals from commercial flocks, ensure high selection intensities and shorter generation intervals together with the accuracy of selection decisions.

SIMULATION STUDY ON USING OPEN NUCLEUS BREEDING SCHEME FOR GENETIC IMPROVEMENT OF WEANING WEIGHT IN BARKI SHEEP WITH SPECIAL EMPHASIS ON FLOCK SIZE