The Lack of Relationship Between Plasma Progesterone Levels and Number of Embryos and Their Survival in the Pig

Abstract

The numbers of corpora lutea (CL) and embryos were altered in normal and superovulated gilts to determine the relationship between the concentration of progesterone in the plasma and numbers of embryos and their survival in the pig. There was no difference in mean level of plasma progesterone between 20 pregnant gilts and 18 nonpregnant gilts on days 5, 6, 9, and 14 after estrus. Superovulation increased (P<0.05) both the number of embryos and level of plasma progesterone over controls. When one ovary was removed on day 5, plasma progesterone levels were lowered and remained lower (P<0.05) than controls until day 19. Progesterone levels and number of CL were correlated (P<0.05) on days 6, 9, 14, 19, and 25 but not on days 85 or 95. Ligation of one oviduct before embryos entered the uterus reduced (P<0.05) the number of embryos present at day 30. The seven experimental treatments produced mean levels of plasma progesterone within groups ranging from 12.2 ng/ml to 27.2 ng/ml and mean litter size from 6.1 to 17.4 embryos at day 30 and 8.6 to 10.8 fetuses at day 100. Ratios of plasma progesterone (ng/ml) per potential embryo varied from 0.83 to 3.1. Levels or ratios of plasma progesterone were not associated with embryonic survival with one exception. Embryonal survival was lower (P<0.05) in the group of superovulated gilts with one ovary removed. The level of progesterone in this group was the same as in controls, but the number of potential embryos was doubled giving the lowest ratio of progesterone to embryos of 0.83. Litter size on day 100 was similar in all treatment groups regardless of the number of embryos allowed to enter the uterus and was lower than in similarly treated gilts on day 30. In a second experiment, all, 75 percent, or none of the embryos were killed at day 30 of pregnancy in 19 gilts. Levels of plasma progesterone were similar in each group to day 60 with 0, 2.5, or 8.5 living fetuses pen gilt. These data demonstrate that either raising plasma progesterone levels to nearly twice normal or reducing them to about half did not influence embryonic survival during the first 30 days of gestation in gilts with normal numbers of ovulations; neither did the number of embryos influence the level of progesterone.