A University of Kentucky reproductive biologist is leading an ambitious four-year study to investigate one of the most pressing, yet largely invisible, problems facing the American swine industry: the mismatch between how many piglets modern sows conceive and how many their uteri can actually support.
Jonathan Pasternak, associate professor in the Department of Animal and Food Sciences (AFS) within the Martin-Gatton College of Agriculture, Food and Environment, received a $650,000 grant from the U.S. Department of Agriculture National Institute of Food and Agriculture (NIFA) to study the developmental impact of uterine crowding in the contemporary sow. The project will run through January 2030 with assistance from the Animal Molecular and Cellular Physiology group, which includes AFS assistant professors Coral Kent-Dennis and Katherine Halloran.
Decades of aggressive genetic selection have pushed litter sizes in commercial swine to historic highs. Top-performing producers now average nearly 16 piglets per litter, with ovulation rates in some genetic lines reaching 40 oocytes - the number of eggs before maturation. But while geneticists have successfully increased the number of embryos a sow produces, uterine capacity has remained stubbornly unchanged.
"Ovulation rate is exceptionally heritable, but uterine capacity can't keep pace," Pasternak said. "We now have sows producing more piglets in a litter than they have nipples to nurse them."
The result for piglets is a condition known as intrauterine growth restriction (IUGR). Unlike traditional runts, which reflect poor genetic potential, IUGR piglets carry the full genetic capacity for efficient growth but are starved of nutrients in the womb. Disruption during this sensitive period has lifelong consequences.
IUGR piglets can represent up to 30% of a contemporary litter but account for the overwhelming majority of pre-weaning mortality. Survivors grow less efficiently and rarely reach market weight.
"From a pure reproductive standpoint, this is arguably the biggest issue facing the swine industry today," Pasternak said.
Researchers will track fetal development at sequential stages of gestation, measuring how and when individual organ systems begin to diverge between crowded and uncrowded environments. The goal is to identify the precise developmental windows during which crowding begins to derail growth. Pasternak's lab will also investigate why a subset of piglets appears naturally resistant to the effects of crowding.
Practical implications for producers
Pasternak emphasized that the goal is not to roll back decades of genetic progress but to inform it.
If the team can identify the genetic and physiological markers that make some piglets more resilient to crowding, producers may eventually be able to select for animals that maintain high litter sizes without the welfare and economic costs of IUGR.
"We're never going back on litter size," Pasternak said. "The goal is to perhaps inform the geneticists of what the limits of uterine capacity really are and find ways to maintain litter size while avoiding these low-quality piglets that won't perform as desired."
The research will be conducted using UK's swine unit and meat laboratory facilities, producing a tissue and sample archive that Pasternak hopes will benefit collaborators across campus.
"IUGR is also a human health issue, so the samples we're generating are likely to be valuable to other researchers at UK and beyond," he said. "We want people to know these resources will be available."
This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2026-67015-45761. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
