Evaluation of vaginal electrical resistance as an indicator of optimal timing for mating in alpacas: a pilot study

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We evaluated the utility of vaginal electrical resistance (VER) as an indicator of the optimal mating time in alpacas. Three parous female alpacas were used in a total of five trials. The alpacas received an intramuscular injection of human chorionic gonadotropin to induce ovulation and synchronize the follicular waves on Day 0. VER was negatively correlated with plasma estradiol-17β and positively correlated with plasma progesterone (P₄). VER on Days 11–12 was significantly lower than that on Day 7. The dominant follicles capable of ovulation were observed by transrectal ultrasonography in four of the five trials on Days 11 and 12. Elevated P₄ levels on Day 7 and their subsequent reduction by Day 12 indicated that Days 11–12 were optimal for mating. Thus, VER measurements appear to be useful for estrus prediction. However, repeated VER measurements in each alpaca are necessary to predict the optimal timing of mating because of the significant interindividual variation in VER.