Scientists may have tapped into the mysteries of the Meishan Chinese pigs' large litter sizes, according to Iowa State University reports.
"We have tremendous potential to increase litter size by as much as three pigs," maintains Steve Ford, Iowa State animal scientist.
For the last 10 years, Ford and his fellow researchers have studied these prolific genetic lines. On average, the Meishan gives birth to 12 to 14 pigs compared to nine to 10 pigs for American breeds.
During the studies, researchers compared the reproductive systems of the Meishan with Yorkshires. They found the placenta surrounding each growing piglet in the Meishan's uterus was smaller than the placenta in the Yorkshire uterus. The placenta moves nutrients and waste products in and out of the uterus and attaches each piglet to the uterine wall.
"What we and the Chinese have unknowingly been doing is breeding for different placental sizes," Ford explains.
"While we tend to select for piglets with high birth weights and growth potential, the Chinese select on number of piglets born. A smaller placenta is more efficient, occupying less of the limited uterine space."
The tests are helping determine if American pigs have the genetic capability to produce large litters from a small placenta. Researchers bred Yorkshire males to females born with large placentas. They then did the same with Yorkshires females born with small placentas.
"In their first litters, the small-placenta breedings farrowed three more pigs than those in the large-placenta crosses," Ford says. "Piglets born to the small-placenta crosses also had small placentas. We repeated the trial, with the same results. The results suggest we, or anyone, can select for litter size."
Now, researchers are developing two distinct herds – one with large placentas and small litters, the other with small placentas and large litters.
"Our goal is to get two populations producing uniform litter numbers," he says. "Once we achieve uniformity, we can work on selecting for small, efficient placentas that grow large, robust pigs."
Ford and his colleagues are studying evidence that the small placenta is a maternal trait. If this holds true, a small-placenta sow could be bred to any superior boar to get large litters. In addition, researchers are looking at whether there are placental-size differences among American hog breeds.