3/16/98

Contacts:
Steve Ford, Animal Science, (515) 294-1436
Brian Meyer, Ag Information, (515) 294-0706

SECRET OF PROLIFIC CHINESE PIGS MAY INCREASE AMERICAN LITTER SIZES

DES MOINES -- Iowa State University animal scientists believe they have unlocked the secret to Chinese pigs' large litters, and they say the information can be used to dramatically increase litter size of American pigs.

"We have tremendous potential to increase litter size by up to three pigs," said Steve Ford, a professor of animal science. Increasing litters by one pig has been estimated to be worth nearly $2 billion to the swine industry.

Ford and graduate research assistants Matt Wilson and Nina Biensen outlined their research at the Midwestern sectional meetings of the American Society of Animal Science and American Dairy Science Association, March 16-18, in Des Moines.

For 10 years, Ford and other ISU researchers have studied the prolific Chinese Meishan pig. The Meishan gives birth to 12 to 14 pigs, compared to 9 to 10 pigs for American breeds. In Iowa, the average litter size is 9.7 pigs.

The researchers compared the reproductive systems of the Meishan with the American Yorkshire breed. They discovered the placenta surrounding each growing piglet in the Meishan's uterus was smaller than the placenta in the Yorkshire uterus. The placenta plays a key role in the exchange of nutrients and waste products into 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 said. "While we have tended to select for piglets with high birth weight and growth potential, the Chinese have long selected on number of piglets born. A smaller placenta is more efficient, occupying less of the limited uterine space. Basically, you get more peas in a pod."

The researchers could identify the most efficient placenta by calculating the ratio of piglet weight to placental weight. The greater the ratio, the more efficient the placenta. "Surprisingly, we found three- to four-fold differences in this ratio within a litter," Ford said. "There were big piglets attached to small placenta, small piglets attached to large placenta and lots in-between."

To find that ratio, the researchers need some thread, a few clips and patience. As a sow gives birth, the researchers catch each piglet as it's born. They clip the umbilical cord in two places and attach matching tags to either end of the clips. The cord is cut between the tags, and the placental end of the cord retracts into the birth canal. When the afterbirth is expelled, the researchers can compare a piglet's weight to its tagged placenta.

The researchers wanted to determine whether American pigs have the genetic capability to produce large litters from small placenta. They bred Yorkshire males and females born with large placentas together, and did the same to those born with small placentas.

"In their first litters, the small-placenta pigs gave birth to three more pigs than those in the large-placenta crosses," Ford said. "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. It's very low-tech."

The researchers are now developing two distinct swine herds &emdash; one with small placentas and large litters, and the other with large placentas and small litters. "Our goal is to get two populations producing uniform litter numbers," Ford said. "Once we achieve uniformity, we can work on selecting for small, efficient placentas that grow large, robust piglets."

They will study evidence that the small placenta is a maternal trait. If so, a small-placenta sow could be bred to any superior boar to achieve larger litters. They also will study whether there are placental-size differences among American breeds.

The Iowa Agriculture and Home Economics Experiment Station funded the research.

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