Relationships between intra-vaginal electrical impedance, estrus behavior, plasma levels of ovarian steroids and the pre-ovulatory luteinizing hormone surge in the prediction of the optimal insemination time in normal cycling pigs

A study aimed at determining the relationships between intravaginal electrical impedance (EI), estrus behavior, plasma concentrations of reproductive hormones (progesterone, estradiol-17p and luteinizing hormone) in prediction of the optimal insemination time was conducted in sixteen normal cycling pigs. Measuring of intravaginal electrical impedance (EI) was carried out throughout the estrous cycle using a 2-electrode impedance-measuring instrument. Blood samples were collected along side with measuring of EI to determine plasma concentrations of reproductive hormones using radioimmunoassay. High values of impedance measured during the diestrus period, and low values of EI coincided with the preovulatory LH-peak at the height of standing heat. Fluctuations in the values of EI were significantly correlated with changes in plasma LH (r = -0. 61) (P < 0.01) during estrus period. EI values began to decrease during proestrus (days 18/19 of the estrous cycle). Average low values of impedance were registered within 36 to 48 hours at times of standing heat. Further, low EI values were significantly correlated to maximum plasma levels of estradiol- 17p (r = -15) (P < 0.05). Plasma progesterone levels also revealed significant association with EI values (r—0.3) (P < 0.05). EI values were also associated with psychosexual behavioral changes and changes in the reproductive tract during estrus period. In conclusion, based on values of EI and the relationships with estrus behavior, plasma levels of ovarian steroids and the pre-ovulatory LH peak; stages of the estrous cycle can readily be estimated, and thus, the optimal insemination time can be predicted.