Cross-fostering piglets between litters is a common practice aimed at improving survival and growth of piglets by equalizing the number of pigs per litter and minimizing weaning weight variation.

Researchers at McGill University conducted an experiment to study the effects of littermate weight on survival, weight gain and suckling behavior of low-birth-weight piglets in cross-fostered litters of two different sizes, according to Malachy Young, researcher at Kansas State University.

The lightweight pigs gained 8.5 pounds during the 21-day lactation period when placed in a small litter with other lightweight pigs. The lightweight pigs gained 7.8 pounds when placed in a large litter with other lightweight pigs.

However, when the lightweight pigs were placed in small or large litters with heavy pigs to create "variable" litters, they gained 9 pounds and 8.3 pounds, respectively. Cross-fostering to create consistent lightweight litters did not improve performance of the light-weight pigs.

A slightly higher number of lightweight pigs did die when reared with larger littermates, however pig numbers in the trial were not adequate to fully test mortality.

Overall, researchers concluded that mortality was not influenced by treatment. Creating lightweight litters did save more lightweight pigs; but this came at the expense of other pigs that then had to be fostered. In the end, this produced no overall improvement in preweaning mortality.