The CIDR-GPG Protocol Improves Reproductive Efficiency in Yaks and Lowers the Body Condition Requirements for Success

Yaks are a critical livestock species for plateau regions; however, their poor reproductive efficiency-characterized by low estrus and pregnancy rates-significantly hampers genetic improvement and industry sustainability. Although synchronization protocols are widely utilized in cows to enhance reproductive performance, research on yaks is limited. Our study systematically evaluated 3 estrus synchronization protocols (CIDR-GPPG, CIDR-GPG, GPPG) on 99 female yaks. The GPPG treatment resulted in the lowest estrus detection (40.63%) and pregnancy rate (15.38%), while the CIDR-GPG protocol significantly improved these rates, achieving an estrus rate of 65.71% and a pregnancy rate of 52.17%. Notably, serum estradiol concentrations were significantly elevated in the CIDR-GPG group compared to the GPPG group. Given the importance of body condition for synchronization outcomes, we analyzed correlation between body measurements and reproductive performance. Among 10 body measurements, chest girth demonstrated the strongest positive correlation with both estrus (R = 0.75) and pregnancy success (R = 0.72). Further logistic regression modeling based on chest girth generated robust predictors for estrus (AUC = 0.934) and pregnancy (AUC = 0.923). Importantly, CIDR-GPG protocol effectively lowered the body condition threshold required for successful reproductive outcomes, indicating that yaks with smaller chest girths can still achieve comparable probabilities of estrus and pregnancy when employing this protocol. Overall, our findings supported the CIDR-GPG protocol as an effective strategy for optimizing yak reproduction and highlighted chest girth as a valuable indicator for predicting reproductive outcomes, providing a practical approach to enhance reproductive management, even in animals with suboptimal body conditions.