How many piglets can modern sows conceive and how many can their uteri actually support? That’s a question University of Kentucky reproductive biologist Jonathan Pasternak is seeking to understand as decades of aggressive genetic selection have pushed litter sizes to historic highs.
Top-performing producers average nearly 16 piglets per litter, the University of Kentucky reports, with ovulation rates in some genetic lines reaching 40 oocytes — the number of eggs before maturation. Although geneticists have successfully increased the number of embryos a sow produces, uterine capacity has remained stubbornly unchanged.
Pasternak is leading a four-year study to investigate this issue through a $650,000 grant from the USDA National Institute of Food and Agriculture to study the developmental impact of uterine crowding in the contemporary sow.
“Ovulation rate is exceptionally heritable, but uterine capacity can’t keep pace,” Pasternak points out. “We now have sows producing more piglets in a litter than they have nipples to nurse them.”
One of the Biggest Issues Facing the Swine Industry
The result is a condition known as intrauterine growth restriction (IUGR) in piglets. 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, he says. 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 preweaning 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 says.
His team will track fetal development at sequential stages of gestation, measuring how and when individual organ systems begin to diverge between crowded and uncrowded environments. In the end, they hope to identify the precise developmental windows during which crowding begins to decrease growth. They will also look into why a subset of piglets appears naturally resistant to the effects of crowding.
Pasternak says he is not trying to roll back decades of genetic progress but to inform it. If the researchers can identify genetic and physiological markers that make some piglets more resilient to crowding, producers may be able to select for animals someday that maintain high litter sizes without the welfare and economic costs of IUGR.
“We’re never going back on litter size,” Pasternak says. “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.”
