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

摘要

The objectives were to quantify direct and correlated responses and to estimate genetic parameters for ovulation rate (OR), number of fully formed pigs at birth (FF), and other production traits following two-stage selection for OR and FF. Three lines were used. Line IOL was derived from a line previously selected eight generations for an index to increase OR and embryonic survival. The other two lines, COL and C, originated from line C_i, the control for the index line. Line IOL had greater OR (4.24 ± .38 ova) and FF (1.97 ± .39 pigs) at Generation 0 than lines COL and C. Lines IOL and COL underwent nine generations of two-stage selection. In Stage 1, all gilts from 50% of the litters with greatest FF were retained and OR were measured on them. Approximately 50% of these gilts were selected on OR in Stage 2. The gilts selected for OR were mated to boars selected from the upper one third of the litters for FF. The two-stage selection was repeated in their progeny. Line C was randomly selected. Each line had 35 to 55 litters by 13 to 18 sires per generation. Ovulation rate was measured by laparotomy 11 d after second estrus. The hypothesis was that selection for OR in a first stage, and on uterine capacity (UC) in a second stage cam increase litter size. An animal model including, depending on the trait, the random additive direct, additive maternal, and common litter of birth effects and fixed effects of generation and sex was used. At Generation 9, differences in mean EBV for OR and FF between lines IOL and C were 6.20 ± .29 ova and 4.66 ± .38 pigs; differences between lines COL and C were 2.26 ± .29 ova and 2.79 ± .39 pigs; and differences between lines IOL and COL were 3.94 ± .26 ova and 1.86 ± .39 pigs. Coefficients for regression of line mean EBV on generation number were .27 ± .07 ova/generation (P .01) for OR, and .35 ± .06 pigs/generation (P .01) for FF in line IOL; .30 ± .06 ova/generation (P .01) for OR, and .29 ± .05 pigs/generation (P .01) for FF in line COL; and .01 ± .07 ova/generation (P > .10) for OR, and .02 ± .05 pigs/generation (P > .10) for FF in line C. The two-stage selection procedure was effective in improving both OR and FF. All the increase in OR was realized as an increase in FF in line COL. Number of fully formed pigs at birth increased more than OR in line IOL presumably because UC was limiting litter size at generation 0 in this line. Correlated responses to selection for OR and FF were decreased age at puberty and increased prenatal loss and birth weight. Inconsistent correlated responses were found for growth and backfat in the two lines selected. Two-stage selection seems to be a promising procedure to improve litter size in swine.