THE EFFECT OF NUMBER OF SOWS AND SERVICE TIME OF BOARS ON SELECTION RESPONSE AND INBREEDING USING AN ANIMAL-MODEL IN A CLOSED NUCLEUS HERD

A simulation study was conducted to examine the influence of population size and number of boars in service per year on overall economic response which was predicted under an BLUP-animal model. The simulated populations should resemble a closed nucleus herd of a pig breeding company. With extension of the sow populations from 60 to 180 sows the overall economic response increased by 32 % and the discounted net economic response increased by 53 %. The main reason for the higher response in larger populations was the higher realized selection intensity which increased by 17 % (24 boars/60 sows versus 24 boars/180 sows). For the same populations, the accuracy of predicted breeding values increased by 8.1 % due to more information available from relatives. The reduction in additive genetic variance showed only a small difference between the populations with different sizes, but when selection response was calculated per 1 % reduction in additive genetic variance the efficiency to exploit genetic variance in larger populations was up to 50 % higher. When sequential culling of boars in the breeding stock was practised, also a reduction in generation interval was found with increased population size. Under an equal size of sow population, a higher number of boars per ar led to an increase in selection response due to a low generation interval (e.g. 6 % when 24 versus 12 boars per year were used in service in a population of 180 sows) and a lower reduction in additive genetic variance (1 % higher genetic standard deviation when 24 boars were used) which more than offset the lower realized selection intensity (-1.5%) and accuracy of selection (-1.0). The expected one generation responses predicted by the conventional approach overestimated the overall genetic merit by 24 % to 46 %. Causes were 8 %-19 % overestimation of accuracy of selection, 13 %-15 % lower genetic standard deviation and 4 %-13 % overestimation of selection intensity under fixed ser-vice time of boars. The rate of inbreeding was very high with 1.36% to 2.04% and depends mainly on the number of boars and the selection strategy of boars. Sequential culling of boars resulted in a high rate of inbreeding due to an increase in variance of family size. In comparison to selection on selection index, under an animal model the use of more boars per year with short fixed service time in a large sow population was recommended.