Uterine capacity in the pig reflects a combination of uterine environment and conceptus genotype effects

Abstract

Prenatal losses in U.S. pig breeds range from 30 to 50%, of which greater than 75% occurs before d 30 of gestation and is thought to result from littermate asynchrony. Numbers of embryos can be experimentally increased to d 30 using superovulation (excess ova shed) and superinduction (transfer of embryos to an already pregnant uterus); however, these females farrow the same number of pigs as untreated controls. These data demonstrate that between d 30 and parturition there are significant additional periods of conceptus loss, which has led to the conclusion that uterine capacity (i.e., the number of conceptuses a sow uterus can accommodate) is the major limitation to litter size in the pig. The special importance of uterine capacity in the pig may result from the noninvasive epitheliochorial type of placentation in this species, making the surface area of attachment between the placenta and endometrium a limiting factor. In devising selection schemes for this trait, one could logically conclude the selection for either longer uterine horns or for a reduced conceptus size should potentially increase litter size in the pig. Researchers have evaluated the impact of differences in prepubertal uterine horn length on subsequent uterine capacity using a unilateral hysterectomy-ovariectomy model but have had modest and variable success in increasing litter size at farrowing. In contrast, results from our laboratory suggest that placental size is moderately heritable and results in consistent increases in litter size of two to three pigs in the Yorkshire breed with little impact on pig birth weight or neonatal viability. This selection of pigs for smaller and relatively more efficient placentae (i.e., the number of grams of fetus that can be supported by 1 g of placenta) seems to provide a useful method for increasing litter size in the pig. A careful evaluation of the physiologic and genetic differences of conceptuses with differing placental efficiencies is necessary if we are to determine specific factors affecting litter size in the pig.