Selection for placental efficiency in swine: conceptus development.

The objective of this study was to evaluate correlated responses in conceptus development and traits physiologically relevant for placental function in swine from a selection experiment that resulted in differences in placental weight (PW) and efficiency (PE = birthweight/placental weight). Generation 3, second parity females from 2 lines with a history of selection on an index predicting either high PE (HPE) or low PE (LPE) were mated within line to produce Generation 4 litters for evaluation at d 30, 50, 70, 90, and 110 of gestation (n = 5/line × d combination). Maternal and fetal traits were analyzed by using a model including the fixed effects of line and gestational age, and the random effect of sire within line. Uterine length was not different between lines at any gestational age, but increased (P = 0.06) from 275.0 ± 23.1 cm at d 30 to 338.3 ± 23.3 cm at d 50, and remained relatively unchanged to d 110. Fetal weight was not different between lines from d 30 to 90, but was less (P = 0.02) in HPE than LPE at d 110 (1,280.6 ± 77.0 vs. 1,551.1 ± 75.3 g, respectively). Crown-rump length was not different between lines from d 30 to 70, but tended (P = 0.09) to be longer in HPE than LPE at d 90 (265.8 ± 8.8 vs. 241.2 ± 10.6 mm, respectively) and was shorter (P = 0.04) in HPE than LPE at d 110 (290.6 ± 5.0 vs. 304.9 ± 4.5 mm, respectively). Placental weight increased in both lines from d 30 to 50, at which point it remained relatively unchanged through the rest of pregnancy, except in LPE that showed a second increase from d 90 to 110. As a result, placental weight was not different between lines from d 30 to 90, but was less (P < 0.01) in HPE than LPE at d 110 (244.6 ± 32.3 vs. 379.2 ± 24.5 g, respectively). Line differences in placental efficiency were not significant at any gestational age. Implantation site length increased slowly for both lines from d 30 to 90, where it remained unchanged to d 110. Implantation site area was greater (P < 0.05) in HPE than LPE at d 30 and 50, but was not different between lines for the remainder of pregnancy. These results suggest that in Western breeds, a reduction in placental weight through selection is not accompanied by compensation in placental nutrient transfer and may result in decreased prenatal survival.