Accuracy of predicting the LH surge and optimal insemination time in Holstein heifers using a vaginal resistance probe.

Monitoring changes in vaginal electrical resistance in association with ovarian follicular development during proestrus may provide an alternative to visual estrus detection of cattle for properly timing insemination. In two experiments 94 Holstein heifers were synchronized into estrus with 2 injections of prostaglandin F(2) alpha (PG) 11 d apart. Beginning 12 h prior to the second PG treatment, blood samples and vaginal resistance measurements were taken every 12 h for 60 h, and then every 6 h for the following 48 h. In both Experiments I and II, the lowest resistance value was found to be highly correlated with the luteinizing hormone (LH) preovulatory surge (r = 0.67 P<0.001). The linear regression of time from PG to the LH surge (Y) on time from PG to the lowest resistance value (x) was defined by Y = 17.49 + 0.71 (x) (R(2) = 0.45). This suggests that the lowest resistance value can predict the LH surge and, therefore, the appropriate time to inseminate heifers relative to the expected time of ovulation. This concept was tested in Experiment II by assigning 50 heifers to be inseminated after PG treatment on the basis of their vaginal resistance pattern, by appointment 66 h after PG, or by visual observation of standing estrus. Conception rates were 58, 50 and 50% in these groups respectively, and did not differ significantly. Overall, these results suggest that measurement of vaginal resistance patterns reliably predict the LH surge and can be successfully utilized to inseminate cattle.