Estrus synchronization and follicular development patterns in jennies

Seasonal variations and breed differences significantly influence reproductive efficiency in jennies, yet optimized, evidence-based estrus synchronization protocols for key Chinese breeds are lacking.

This study aimed to compare the efficacy of three hormonal protocols on estrus synchronization, and to characterize follicular development patterns in DeZhou and Xinjiang jennies across different seasons.

In two experiments (April-May, n = 94; July-August, n = 144), 238 non-pregnant jennies received one of three treatments: GnRH-PGF₂α with two injection timings (Treatment Group 1 and 2), or a Controlled Internal Drug Release (CIDR) insert combined with GnRH-PGF₂α (Treatment Group 3). Estrus, ovulation, and conception rates were recorded. Follicular dynamics were assessed daily via ultrasonography in 125 jennies (68 DeZhou, 57 Xinjiang).

During April-May, TG3 achieved a higher total estrus rate (85.0% vs. 53.0% and 72.5% in TGs1 and 2, respectively, P < 0.05) and conception rate (81.3% vs. 66.7% and 71.4%, P < 0.05). Reproductive outcomes were superior in April-May versus July-August (P < 0.05). Xinjiang jennies exhibited a faster follicular growth rate (5.52±2.56 mm/d) compared to DeZhou jennies (4.32±1.64 mm/d, P < 0.05), while DeZhou jennies had larger pre-ovulatory follicles (44.99±1.70 mm vs. 31.29±0.95 mm, P < 0.05). No significant lateral preference in ovulation was observed (left ovary: 54.2%, right ovary: 45.8%).

The CIDR+GnRH-PGF₂α protocol applied in April-May most effectively synchronized estrus and improved conception rates in jennies. Follicular growth patterns differed between breeds, informing breed-specific insemination timing. No ovarian lateralization was detected.