Index Selection for Components of Litter Size

University of Nebraska 1995 Swine Day Report. Pigs per sow per year has gradually, but steadily, increased in the U.S. during the last 15 to 20 years. More efficient use of better maternal breeds and lines, crossbreeding systems that efficiently utilize heterosis, and improved management and diets are causes for most of this improvement. Genetic selection to further increase reproductive traits such as litter size and litter weaning rate has recently been implemented and may explain some of the increase in sow productivity in the latter years. However, efficient programs to improve sow reproduction have not been in place long enough to cause much change in the U.S. pig herd. Continuous application of efficient selection programs in seedstock herds will be necessary to further improve reproductive performance. However, reproductive traits generally have low heritabilities and other traits also must be emphasized in selection programs. Selection accuracy has been increased through the use of computers and genetic analyses to estimate breeding values, but even when these procedures are used, the annual rate of change will not be great. Therefore, there is a need for procedures to speed the rate of improvement from genetic selection for reproductive traits. Such a procedure was tested experimentally at the University of Nebraska. Selection was for an index of ovulation rate and embryonic/fetal survival rate measured at 50 days of gestation. The experiment was described, and results of the first five generations of selection were reported, in the 1988 Nebraska Swine Report. Selection was continued for another five generations and an additional generation with random selection was produced to evaluate the lines. The purpose of this article is to report the results of this index selection on ovulation rate and on litter sizes at 50 days of gestation and at birth and to briefly discuss the potential application of this selection method.