The Nebraska Large White-Landrace synthetic has undergone index selection for litter size since the initiation of the lines in 1981. Selection for litter size was done using an index of ovulation rate and embryo/fetal survival to 50 d of gestation. In the litter size index (LSI) line, ovulation rate and embryo/fetal survival were measured by laparotomy at 50 d of gestation on all females mated from each generation. The AO highest indexing of approximately 150 females were selected and farrowed. Males were selected from the 15 highest indexing dams and mated to gilts to produce the next generation. The control (CLw) line was maintained with random selection of one male per half-sib family and one female per full-sib family. Line size was maintained at AO litters by 15 males in each line. Inbreeding levels are currently A.A6% in the LSI line and 2.71% in the CLw line. Traits measured included o vulation rate (OR), embryo/fetal survival (ES), number of fetuses at 50 d of gestation (FETS), fetal loss from 50 d of gestation to parturition (LOSS), total number of fully formed piglets born (TNB), and number of mummified piglets (MUMM). Heritability estimates obtained by parent-offspring regressions on A29 pairs were: OR (.10 ± .09), FETS (.32 + .1A), ES (.13 + .11) and LOSS (.A2 + .20). Heritability estimated for TNB (.22 ± .10) was based on 537 parent-offspring pairs. Genetic correlations were also calculated using parent-offspring regression. OR was positively correlated with FETS (.21 + .A2) and with TNB (.03 + .AA), however, negatively correlated with ES (-.30 + .53) and LOSS (-.08 _+ .A5). FETS was positively correlated with TNB (.18 +_ .31) and highly positively correlated with ES (.83 ± .13). Expected response to selection for alternative selection strategies is presented.

S. M Neal, R. K Johnson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 228–233, 1986
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