Associations of accelerometer variables with the timing of ovulation in grazing beef heifers

Accelerometers can improve precision in estrous detection, but validation is needed for use in beef operations. The hypotheses were (1) the interval from the estrous onset to ovulation is shorter when estrus is detected by the accelerometer software than when detected by visual observation, and (2) the accelerometer variables are associated positively with the timing of ovulation. Beef heifers (n = 117) fitted with ear-tag accelerometers that detect rumination and activity were subjected to an estrous synchronization protocol. Estrus (hour 0) was monitored every 2 h using estrous detection patches. Ovulation was checked every 3 h, starting at 18 h, by transrectal ultrasonography. The interval from estrous onset and peak of estrus in the accelerometer software to ovulation was 25.25 ± 0.55 h and 19.07 ± 0.49 h, respectively. The estrous duration, heat index, percentage of increase-activity, peak estrus, proestrus, and the percentage of increase-heat variables were associated positively with either the interval from estrous onset to ovulation or the interval from the peak of estrus to ovulation (P < 0.05). Multivariate analysis demonstrated that when heifers were separated into the top 10th and bottom 10th percentiles of the interval from the estrous onset to ovulation, there was overlap between classes. In conclusion, the interval from estrus to ovulation is shorter in the accelerometer software than visual observation, and accelerometer variables are significantly associated with the time of ovulation but do not separate heifers into different classes of ovulation time without overlapping. Future studies are necessary to determine an optimal breeding window based on the ovulation time.