Progesterone therapy during early pregnancy and embryonal survival in primiparous weaned sows

The experiment was conducted in primiparous sows to determine whether progesterone supplementation during early pregnancy could improve embryonal survival and to confirm a functional relationship between progesterone concentrations in the postovulatory period and embryonal survival. Thirty-four primiparous sows were fed for ad libitum intake from farrowing to d 21 of lactation, restricted to 50% of ad libitum intakes between d 22 and 28, weaned on d 27, and then fed for ad libitum intake from weaning until d 28 of gestation. The number of pigs per litter was standardized to nine within 48 h after farrowing. After mating, sows were allocated either to be treated with ethyl oleate (control group) or to receive progesterone (P4) therapy (progesterone group) as 2 mg of P4/kg(.75) i.m., every 12 h from 36 to 96 h after onset of standing estrus. There was no difference in the weaning-to-estrus interval (123.5 +/- 5.7 vs 123.1 +/- 5.9 h) and number of corpora lutea (18.1 +/- .4 vs 18.3 +/- .5) between the control and progesterone sows (P > .05). Compared with a more gradual increase in control sows, plasma progesterone concentrations in progesterone sows increased immediately from .6 +/- .1 ng/mL at 36 h after onset of standing estrus to 6.3 +/- .5 ng/mL (P < .001) 12 h later and remained high. Total number of embryos, total embryonal survival rate, number of viable embryos, and viable embryonal survival rate on d 28 of gestation in control sows were greater than in progesterone sows (13.3 +/- 1.0 vs 8.5 +/- 1.1; 73.1 +/- 4.7 vs 47.3 +/- 5.4%, P < .005; 12.6 +/- .9 vs 7.1 +/- .9, and 69.5 +/- 4.2 vs 39.7 +/- 4.8%, P < .0005; respectively). Embryonal survival in control sows was comparable to that in previous experiments using the same primiparous sow model, and the relationship between plasma progesterone and embryonal survival seemed similar. However, using the treatment schedule adopted, progesterone not only failed to reverse the presumed detrimental effect of increased catabolism in the late lactation period on embryonal survival, but also adversely affected the number of embryos at d 28 in 7 out of 15 progesterone-treated sows.