Comparative aspects of placental efficiency.

Litter size is often proposed as the trait that could have the greatest impact in improving reproductive efficiency of pigs. Efforts to select directly for increased litter size have generally been unsuccessful and highly variable. As a result, several attempts have been made to identify critical physiological components that control litter size, with the underlying assumption that augmenting these components would improve this important trait. One attempt at improving physiological components has involved the selection of animals for increased uterine capacity, as measured by the number of fetuses or piglets that a female can carry successfully to term. Recent evidence indicates that one critical component of the uterine capacity in pigs is placental efficiency, or the body weight of a piglet divided by the mass of its placenta. It is easy to determine the average placental efficiency in a litter, but variation among conceptuses within a litter for this trait can be substantial, leading to the conclusion that placental efficiency is an individual conceptus trait. It is suggested that the limited success of selection for an increased uterine capacity results, at least in part, from a misguided view that 'uterine capacity' is strictly a maternal trait. Uterine capacity is better defined as the mass of placental tissue that a pregnant female can support to term, and involves phenotypic variation in both the dam and her offspring. This definition of uterine capacity allows maximization of both uterine size and placental efficiency in future attempts to increase litter size in pigs.