Effect of Dexamethasone, Methallibure and Fetal Decapitation on Porcine Gestation

Abstract

The role of the fetal brain in the initiation of parturition was investigated in sows and gilts. In the first experiment, 16 pregnant crossbred sows were assigned in equal numbers to each of four groups. Treatment was as follows: none (controls), intramuscular injection of 100 mg of dexamethasone/day on days 101 to 104 of gestation, feeding of 150 mg of methallibure/ day from day 95 to termination of pregnancy, or treatment with both dexamethasone and methallibure. Analysis of the resulting lengths of gestation: control, 115.0; dexamethasone, 110.5; methallibure, 117.3; dexamethasone + methallibure, 113.7 days, revealed that dexamethasone induced premature parturition (P<.01) but methallibure delayed (P<.01) both spontaneous and dexamethasone-induced parturition to a similar extent. Weight of the testes was reduced (P<.01) in piglets from methallibure-treated sows necropsied within 12 hr after birth, but the weight of the adrenals, thymus, thyroid and ovaries was not affected by any of the treatments. In all groups of gilts progestin concentrations in maternal serum appeared to decrease gradually between day 6 and day 2 prepartum followed by a rapid decline to day of parturition. Thirty pregnant gilts were assigned in equal numbers to one of six treatment groups of experiment 2. Between days 35 and 40 of gestation, all the fetuses in half of the gilts were decapitated and the remaining gilts were subjected to sham surgery. The 15 gilts subjected to each type of surgery were then assigned to receive no further treatment, injections of dexamethasone or injections of dexamethasone and feed containing methallibure (five gilts/subgroup). Dosages and treatment schedules were as in experiment 1. The lengths of gestation of the gilts subjected to sham surgery were similar to those in experiment 1 and were as follows: control, 114.3; dexamethasone, 109.6; and dexamethasone + methallibure, 112.2 days. Following fetal decapitation, none of the gilts farrowed by day 120 of gestation and were necropsied. Live decapitated fetuses were present in one control and two dexamethasone-treated gilts. Of the remaining 12 gilts, two contained dead fetuses estimated to have died after day 80, six contained only mummified debris of concep-tuses and remnants of conception were not discernible in four gilts. Mummified debris was also present in those uteri containing live or dead fetuses. Although serum progestins in the gilts subjected to sham surgery declined rapidly in the last 2 days prepartum, progestin levels observed in gilts with decapitated fetuses or mummified debris remained at late gestation levels until day 120. Apparently a live, intact fetus is required for both spontaneous and dexamethasone-induced parturition.