Control of Estrus in Sheep: Protocol Architectures, Seasonal Effects, and Breed-Specific Reproductive Performance

Abstract

Abstract

Sheep play a crucial role in strengthening food security in different regions, where climates are often variable and resources can be limited. The importance of these animals includes nutrition and income. For sustainable production, manipulating estrous cyclicity is a pivotal, particularly in tropical and dry climate regions. Given the variation in reproductive performance in various breeds with seasons, and management practices, it is difficult to select effective protocols at all times. There is a need for in-depth knowledge on efficiency of available protocols. This review critically synthesizes current estrus synchronization strategies in sheep, integrating physiological principles with comparative evaluation of hormonal molecules, protocol architectures, and reproductive performance data. Progesterone-based systems remain the cornerstone of synchronization, particularly when combined with equine chorionic gonadotropin (eCG), because they simulate luteal activity and promote follicular recruitment during seasonal anestrus. However, prolonged progestogen exposure may induce persistent follicles and compromise oocyte quality, whereas short-term protocols (5–7 days) frequently achieve comparable or improved fertility while reducing hormonal exposure. Gonadotropin-releasing hormone (GnRH)-based approaches improve control of ovulation timing and are particularly useful for fixed-time artificial insemination (FTAI), although their effectiveness declines during deep anestrus without exogenous gonadotropic support. Prostaglandin-only systems provide cost-effective synchronization in cyclic ewes, but their reliability is limited outside the breeding season. Emerging innovations, including recombinant gonadotropins and nano-formulated hormone delivery systems, aim to enhance endocrine precision while addressing welfare, immunogenicity, and regulatory concerns associated with conventional eCG use. Across studies, variation in outcomes reflects the complex interaction among endocrine regulation, follicular dynamics, luteal competence, breed-specific ovarian responsiveness, and seasonal neuroendocrine suppression. Therefore, current evidence suggests that synchronization success depends less on protocol complexity than on physiological compatibility with breed characteristics, seasonal reproductive status, and production objectives. Future research on precision endocrine modulation, reduced hormonal exposure, ethical standardized outcome reporting is warranted.