Efficacy of Two Estrus Synchronization Protocols in Crossbred Gyr Dairy Cows and Their Relationship with Heat Stress in the Peruvian Tropics

The performance of estrus synchronization protocols may be influenced by stressful environmental conditions, even in tropically adapted cattle. This study evaluated the efficacy of two synchronization protocols for fixed-time artificial insemination (FTAI) in crossbred Gyr dairy cows and their relationship with heat stress under Peruvian tropical conditions. Multiparous lactating cows under semi-intensive systems were synchronized and divided into two groups: the conventional protocol (PC9) and the prolonged proestrus protocol (J-Synch6). Estrus manifestation, pregnancy rate, and their association with the temperature–humidity index (THI) were analyzed across seven phases of the FTAI program (73 days). In PC9 cows, no significant effect of the number of hours in each heat stress category according to THI was found. Conversely, in J-Synch6 cows, more hours in Category 0 (THI < 72) during Phases 1 to 4 (follicular emergence to preovulatory follicle) had a positive effect on the estrus manifestation, whereas in Phase 3 (follicular growth), higher exposure to Category 3 (THI ≥ 83) had a negative effect on estrus manifestation. Both protocols demonstrated similar estrus and pregnancy rates, although heat stress affected estrus rates in J-Synch6, but not pregnancy. The choice of protocol should account for heat stress risks during specific periods of the year to maximize success.

This research was funded by the Project "Mejoramiento de la disponibilidad y acceso del material genético mediante el uso de técnicas de biotecnología reproductiva en ganado bovino tropical en las regiones de San Martín, Loreto y Ucayali", with CUI N° 2338934 of the Instituto Nacional de Innovación Agraria. We also express our gratitude to the Vice-Rectorate for Research at the "Universidad Nacional Toribio Rodríguez de Mendoza de Amazonas" for financing the Article Processing Changes (APC).