Layers of open nucleus breeding scheme in relation to the genetic improvement of weaning weight in Barki sheep : a simulation study

الملخص EN

The present study investigated the expected genetic gain of weaning weight by changing the stratification of the open nucleus breeding scheme from two tires system (ON2) to three tires one (ON3).

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 ON3 system consisted of a nucleus flock attached with five multiplier flocks and each of them linked to one commercial flock, whereas ON2 system was constructed by simulating a nucleus flock associated with five commercial flocks.

the open nucleus breeding scheme was also simulated to allow for the exchange of superior sires among tires in both directions.

three 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 indicated that breeding values of sires, dams and offspring as well as average weaning weight tended to increase in ON3 and ON2 systems as generations advanced from the first (G1) to the third generation(G3) in nucleus, multiplier and commercial flocks.

In both ON2 and ON3 systems, there was a consistent trend for the annual genetic gain of average weaning weight to be the highest in G2 and decreased in G3.

Selection for weaning weight for three generations in ON3 system increased average weaning weight from 18.91 kg to28.67 kg (by 51.6%) in the nucleus flock, from 17.85 kg to 26.38 kg (by 47.8%) in multiplier flocks and from16.33 kg to 23.17 kg (by 41.9%) in commercial flocks.

the corresponding values for ON2 were from 19.02 kg to28.35 kg (by 49.1%) in the nucleus flock and from 16.33 kg to 25.17 kg (by 54.1%) in commercial flocks.

these results clearly indicate that the increase in the annual genetic gain of weaning weight happened in the commercial flocks of ON2 system was much higher (54.1% vs 41.9%) than that occurred in the commercial flocks of ON3 system.

Moreover, the annual genetic gain for average weaning weight obtained in the commercial flocks of G2 in the ON2 system was three times as much as that occurred in ON3 one (31.7% and10.2%, respectively).

the respective values in G3 were found to be 28.8% and 17.1% for ON3 and ON2, respectively.

it is quite clear that the improvement of weaning weight not only was increased in the commercial flocks when the two tires system was applied but also reached faster in the second generation while it delayed to the third generation in case of the three tires system.

there is certainly a considerable increase in the genetic response for both ON2 and ON3 systems, however, the practical importance of higher and faster genetic gain, low cost, simplicity and ease of operation might lead to recommending the ON2 system for genetic improvement of weaning weight in Barki sheep.