Over-crowding in uterus harms fetal pig health

Sows given hormones to “super ovulate” are more likely to lose embryos and fetuses in the first 40 days of gestation than untreated pigs, according to a recent study by scientists at Wageningen University in the Netherlands. In a paper published online in April by the Journal of Animal Science, van der Waaij et al. describe how hormonally-induced overcrowding in the uterus may harm fetal health in large white gilts.

The research team dissected the uteri and ovaries of pregnant pigs to investigate if super ovulation produced an increased number of vital fetuses. They found that although there were more fetuses after super ovulation, overcrowding during development had negative consequences for the health of the fetuses. Average fetal weight decreased by almost one-third in pigs with an increased ovulation rate. This may be due to a lack of growing space, nutrients, and a beneficial hormone called estradiol.

The increased ovulation rate also resulted in an increase in poor-quality embryos. These embryos were more likely to be slow to develop, causing an increase in within-litter size variation. Greater numbers of poor-quality fetuses could also harm the survival rates of the stronger fetuses because all fetuses compete for the same resources in utero.

While the researchers studied super ovulation in large white pigs, Liesbeth van der Waaij, lead author of the paper, believes the findings could apply to fetal development in many different breeds.

“I can imagine that the more proliferous breeds will run into problems at a higher ovulation rate than others, but [uterine] space will [eventually] become limiting in all breeds,” said van der Waaij.

Although there was an increase in fetuses after hormonally-induced super ovulation, the researchers caution pig breeders to think about quality over quantity.

“Selection for ovulation rate is not the way to increase litter size. The number of healthy and vital piglets born seems to be better selection criteria. We would recommend [pig breeders] select on the 'end product.' That is, select for the number of healthy piglets born, rather than on aspects of the process leading to that, such as [selection for] ovulation rate,” van der Waajj said.

The researchers do not know whether the vital fetuses observed at day 40 would survive until birth. The researchers point out that from an evolutionary perspective, mortally among unhealthy fetuses in an over-crowded uterus is beneficial for the piglets born alive. Less competition with fellow piglets after birth for food means a greater chance of piglet survival.

Van der Waaji said future studies should investigate fetal health after 40 days of pregnancy. Researchers could let sows carry the super-ovulation pregnancy to term to determine what happens. Van der Waaij’s study used gilts, but she believes the results could also be interesting in second or third parity sows and other pig species.

The report by van der Waaij et al. is entitled “Effect of excessive, hormonally induced intrauterine crowding in the gilt on fetal development on day 40 of pregnancy.” It was first published online by the Journal of Animal Science in April, 2010. Read it in full at http://journalofanimalscience.org/

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Madeline McCurry-Schmidt
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