Responses to 14 Generations of Selection for Components of Litter Size
1998 University of Nebraska-Lincoln Extension Swine Report. Eleven generations of selection for increased index of ovulation rate and embryonal survival rate, followed by three generations of selection for litter size, were practiced. Laparotomy was used to count corpora lutea and fetuses at 50 days of gestation. High indexing gilts, approximately 30 percent, were farrowed each generation. All gilts in these litters were mated to boars selected from litters of gilts in the upper 15 percent of the distribution for index. Selection from generation 12 to 14 was for increased number of fully formed pigs. Replacement boars and gilts were selected from the largest 25 percent of the litters. Total response in the selected line compared to the control was approximately 6.7 ova, 3.9 fetuses, 3 fully formed pigs, and 1.4 live pigs (P < .01) at birth. Ovulation rate and number of fetuses had positive genetic correlations with number of stillborn and mummified pigs, which increased with selection for the index. Approximately 77 percent of the increase in fetuses was represented by a pig at birth, and 36 percent of the increase was a live pig. Average pig birth weight declined as litter size increased. Smaller pigs, and the higher rate of inbreeding in the select line, may have contributed to greater fetal losses in late gestation, greater number of stillborn pigs and lower preweaning viability in the select line. Phenotypic variation in litter size and its component traits is high. Heritabilities were between 10 and 25 percent; sufficient genetic variation exists to increase litter size with selection. Response in total born per litter was approximately 15 percent greater than response predicted from direct litter size selection. This is probably not enough to justify implementation of this procedure in industry breeding programs. Because of undesirable genetic relationships between ovulation rate and the number of stillborn and mummified pigs and decreased birth weight with increased litter size, genetic improvement programs should emphasize live born pigs and perhaps weight of liveborn pigs in selection programs.