Direct and correlated responses to two-stage selection for ovulation rate and number of fully formed pigs at birth in swine

Our objectives were to estimate responses and genetic parameters for ovulat ion rate, number of fully formed pigs at birth, and other production traits following two-stage selection for increased ovulation rate and number of f ully formed pigs. Eight generations of selection were practiced in each of two lines. One selection line was derived from a line that previously selec ted eight generations for an index to increase ovulation rate and embryonic survival (the IOL pigs). The other selection line was derived from the uns elected control line of the index selection experiment (the COL pigs). The control line (C) was continued with random selection. Due to previous selec tion, Line IOL had greater ovulation rate (4.24 +/- 0.38 and 4.14 +/- 0.29 ova) and litter size (1.97 +/- 0.39 and 1.06 +/- 0.38 pigs) at Generation 0 of two-stage selection than did Lines COL and C. In Stage 1, all gilts fro m 50% of the largest litters were retained. Approximately 50% of them were selected for ovulation rate in Stage 2. Gilts selected for ovulation rate w ere mated to boars selected from the upper one-third of the litters for lit ter size. At Generations 7 and 8, differences in mean EBV for ovulation rat e and litter size between Lines IOL and C were 6.20 +/- 0.29 ova and 4.66 /- 0.38 pigs; differences between Lines COL and C were 2.26 0.29 ova and 2. 79 0.39 pigs; and differences between Lines IOL and COL were 3.94 +/- 0.26 ova and 1.86 +/- 0.39 pigs. Regressions of line mean EBV on generation numb er were 0.27 +/- 0.07 ova and 0.35 +/- 0.06 pigs in Line IOL; 0.30 +/- 0.06 ova and 0.29 +/- 0.05 pigs in Line COL; and 0.01 +/- 0.07 ova and 0.02 +/- 0.05 pigs in Line C. Correlated responses were decreased age at puberty an d increased number of pigs born alive, number of mummified pigs, prenatal l oss, and individual and litter birth weight. Two-stage selection for ovulat ion rate and number of pigs per litter is a promising procedure to improve litter size in swine.