Prostaglandin-induced luteolysis and estrous expression in dairy heifers: applications of ovulation prediction based on automated activity monitoring system

This study evaluated the effects of different prostaglandin F_2α (PG) regimens on luteolysis, estrous behavior, and ovulation timing in dairy heifers using an automated activity monitoring (AAM) system. Eight days after presynchronized ovulation (designated as Day 0), twenty-seven heifers were assigned to one of three PG regimens, administered 24 hr apart: a single standard PG dose of 500 µg followed by saline (one standard dose, OSD, n=8), two low PG doses of 375 µg and 250 µg (two low doses, TLD-1.5, n=8), or two PG doses of 250 µg each (TLD-1, n=8). All heifers experienced complete luteolysis and ovulation after PG treatments, however, heifers in the TLD-1.5 and TLD-1 groups exhibited delayed progesterone (P₄) decline, prolonged luteolysis, later estrus onset, and lower peak value of the estrous alarm (PA) than those in OSD. Despite these variations, the interval from estrus to ovulation remained stable, confirming AAM's reliability for predicting suitable artificial insemination (AI) timing. The delayed luteolytic patterns induced by each regimen affected estrous expressions, as the serum P₄ concentration on Day 1 was positively correlated to the timing of highest estradiol (E₂) as well as ovulation but negatively correlated to PA. The ability of AAM to consistently predict ovulation, regardless of variations in luteolysis, ensures its practical value in AI programs. These findings highlighted the importance of optimizing PG regimens while reinforcing the role of AAM in reproductive management.