Maternal Recognition of Pregnancy in Swine. I. Minimal Requirement for Exogenous Estradiol-17β to Induce Either Short or Long Pseudopregnancy in Cycling Gilts1

Abstract

Five experiments were conducted to determine the minimal requirement for estradiol-17 beta (E2) injections to induce either short pseudopregnancy (SPP) or long pseudopregnancy (LPP) in cycling gilts. In experiments 1 through 5, E2 was injected i.m. on combinations of days between 11 and 25 days postestrus. Exogenous E2 on Days 12 and 13 or on Days 12 through 19 was optimal for induction of SPP or LPP, respectively. The duration of E2-induced diestrus was clearly demarcated between SPP (n = 73, duration 23-35 days) and LPP (n = 23, duration > 50 days). A sixth experiment was conducted to determine the minimum dose of intrauterine E2 required to induce SPP, and these gilts received intrauterine infusions of 0, 4, 40, or 400 micrograms E2 per 24 h on Days 12 and 13 postestrus. Pseudopregnancy was induced in 0 of 12, 1 of 4, 1 of 11, and 4 of 7 gilts in the treatment groups, respectively. These data suggest that uterine exposure alone is not sufficient to induce SPP. The present results indicate that the optimal signal for inducing LPP in unmated cycling gilts, and perhaps also for maternal recognition of pregnancy in mated gilts, may occur in two phases with continuous exposure to E2 being required from Day 12 to Days 17-19.