Ovarian follicular and luteal dynamics during natural and hormonally induced estrus in Arunachali yak (Bos grunniens)

This study investigated follicular growth patterns, corpus luteum dynamics and endocrine profiles during natural and hormonally induced estrous cycles in Arunachali yak maintained at the ICAR–NRC on Yak, Nyukmadung, Arunachal Pradesh, India. Thirty-six apparently healthy, cyclic female yaks of second to fourth parity were equally assigned to natural estrus (Group I) or hormonally induced estrus using a modified Ovsynch protocol (Group II). Daily transrectal ultrasonography was performed throughout one estrous cycle to characterize follicular waves and luteal dynamics, along with serial endocrine profiling. Three follicular-wave patterns were observed in natural cycles and four in induced cycles. A two-wave pattern predominated in natural estrus (66.67%), whereas a three-wave pattern predominated following hormonal induction (77.78%). The mean estrous cycle was significantly longer in induced than natural cycles (22.50 ± 0.35 vs. 20.00 ± 0.70 days). The first follicular wave was the longest in both groups, with successive waves showing progressively shorter durations. Corpus luteum size differed significantly between groups, indicating altered luteal dynamics following hormonal induction. Endocrine profiling further showed higher progesterone (5.75 ± 0.13) during induced cycles, increased estradiol (29.20 ± 3.19) at subsequent estrus and the third follicular peak, increased IGF-1 (124.63 ± 2.97) during the mid-and late-luteal phases and elevated cortisol (3.47 ± 0.25) during the early-to-mid luteal phase. Overall, hormonal induction altered follicular-wave patterns, cycle length, luteal development and endocrine dynamics in Arunachali yak. These findings facilitate the optimization of estrus synchronization and fixed-time artificial insemination (FTAI) protocols, thereby enhancing ovulatory precision and overall reproductive performance in yaks (Bos grunniens).