Prediction of the estrous cycle and optimal insemination time by monitoring vaginal electrical resistance (VER) in order to improve the reproductive efficiency of the Okinawan native Agu pig

The present study was conducted to determine whether vaginal electrical resistance (VER) can be exploited to improve the low reproductive efficiency of the rare Okinawan native Agu pig, in which estrous signs are difficult to ascertain by visual observation. The lowest VER (272.0 ± 12.4 units, n = 5) and the preovulatory LH surge were detected at 57.6 ± 5.3 and 36.8 ± 9.6 h before the onset of estrus, respectively. The initiation of gradual increase in VER was found after 9.6 ± 4.7 h following the peak LH, and the higher levels of VER were plateaued during the luteal phase. These VER fluctuations were correlated with changes in plasma LH (P < 0.05) and progesterone (P < 0.001), but not estrogen. Moreover, the conception rate (41%, n = 32) was dramatically improved by artificial insemination at 24 and 34 h after the beginning of the VER increase when compared with insemination at the conventional time (12 and 24 h after detection of estrus, 20%, n = 45), widely used in commercial pigs (P < 0.05). These data suggest that VER fluctuation can be used to estimate the stage of the estrous cycle, and the scheduling artificial insemination according to increase in VER as an index for the preovulatory LH surge could improve Agu reproductive efficacy.