Selection for increased index of ovulation rate (OR) and embryonic survival (ES) at 50 d of gestation (Index line) and random selection (Control line) were practiced for 10 generations. Selection was based on
 dam's index (I) and selection rates were approximately 40/160 for females and 15/160 for m»W Laparotomy at d 50 of gestation was used to expose ovaries and the uterus and index was calculated from counts of corpora lutea and fetuses (F). The index was constructed to maximize response in F. Because the optimum economic weight for OR was the ES mean and the weight for ES was the OR mean, the index was changed after five generations to reflect changes in means from generation 0 to 5. This new index was used to select parents of generations 6 to 10. The mean parents cumulative selection differential was 96.5 points at generation 5 for the
 first index and an additional 130.6 points from generations 6 to 10 for the second index, for a total of 227.1 points. Regressions on mean parents' cumulative selection differential for generations 0 to 5 were .23 ± .04 (I) .024 ± .004 (OR), and .012 ± .003 (F). For generations 6 to 10 these regressions were .256 ± .027 (I), .033 ± .003 (OR), and .013 ± .001 (F). ES decreased an average of -.035% per cumulative selection differential during 10 generations. Number of fully formed pigs at birth increased .108 ± .049 pigs per generation in the
 select line compared to the control, but the increase in number bom alive was not significant.

D. Casey, T. A Rathje, R. K Johnson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 315–318, 1994
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