Selection for litter size may result in an increase in uterine crowding due to a faster increase in ovulation rate than in litter size. Increased ovulation rate does not result in a proportionally increased number of piglets born alive. In this study, the effect of ovulation rate on vitality characteristics of fetal-placental units at d 40 of pregnancy was investigated. For this, 43 Large White gilts were treated with hormones to induce superovulation. Average ovulation rate was 45.16; average number of vital fetuses at d 40 of pregnancy was 17.09 that weighed an average of 11.26g; their placenta weighed on average 31.88g; and they occupied 11.69cm of the uterus. Loss in oocytes (i.e., that did not result in a vital fetus at d 40) increased with increasing ovulation rate and occurred before and after implantation. With respect to the vital fetuses, increased ovulation rate resulted in decreased fetal and placental weight and decreased length of the area in the uterus that was occupied by the placenta. Strong correlations existed between placental and fetal weight, and placental weight and length. Fetal-placental characteristics were weakly correlated to distance to the implantation sites of neighboring fetuses, a measure of crowdedness with fetal weight to 0.16. Increased ovulation rates, but more specifically increased late mortality rates, have negative effects on the remaining vital fetuses with respect to the fetal and placental weight and length of the implantation site. The most extreme effect was on placental weight, in which a uterus with <10 cases of late mortality was on average 25% greater than placental weight in a uterus with >18 cases of late mortality. Furthermore, increased ovulation rates resulted in decreased within litter variation for fetal and placental weight. At increased ovulation rates, the number of live fetuses remained similar, but placental development is impaired and the growth of the fetus is retarded compared with reduced ovulation rate, with effects likely lasting into adult life. 

For more information the full article can be found at http://jas.fass.org/