Date of this Version
Litter size continues to be an important economic variable in pig production. Two determinants of litter size are ovulation rate and uterine capacity, where uterine capacity is defined as the maximum number of pigs a female can carry to parturition. When the number of fertilized ova exceeds uterine capacity embryo/fetal losses during gestation reduce litter size to that sow’s uterine capacity. This experiment tested whether litter size can be increased by direct selection for ovulation rate and uterine capacity. It was accomplished by selecting for ovulation rate (OR) and number of fully formed pigs at birth (FF) in two stages. All gilts from 50% of the largest litters were selected in Stage 1, and then 50% of these gilts were selected on OR in Stage 2. Litter size at birth in gilts with high ovulation rate was considered a measure of their uterine capacity. Selected gilts were mated to boars selected from the upper one third of the litters for FF. Selection in each of two lines for eight generations was practiced. One of the lines, designated IOL, was started from a line previously selected for increased ovulation rate and embryonic survival, and thus began with a base of high litter size. The other line, designated COL, began from an unselected population. Responses in these lines were compared to those in a randomly selected control line (Line C). The genetic increases per generation for OR and FF were .27±.07 ova/generation (P<.01) and .35±.06 pigs/generation (P<.01) in line IOL and .30±.06 ova/ generation (P<.01) and .29±.05 pigs/ generation (P<.01) in line COL. In previous experiments, only 25 to 50% of the increase in OR was realized as a pig at birth. In this experiment OR and FF increased equally, indicating that increased litter size resulted in approximately equal increases in both ovulation rate and uterine capacity. Furthermore, the responses were similar in both lines indicating that the selection procedure will work effectively in populations with varying base levels of ovulation rate, uterine capacity and litter size. The selection procedure is still not practical in most breeding programs because the surgical procedure of laparotomy or laparoscopy is used for accurate measurement of OR. However, if a noninvasive procedure to measure OR is developed, this procedure can be effectively applied in industry breeding programs. Other changes that occurred as a result of this selection were decreased age at puberty, increased number of pigs born alive, and increased number of stillborn and mummified pigs. Two-stage selection for FF and OR is an effective procedure to improve litter size in swine.