Progesterone-based timed AI protocols for Bos indicus cattle I: Evaluation of ovarian function

Area of Science:

• Reproductive biology
• Veterinary science
• Animal science

Background:

• Progesterone-based protocols are crucial for estrous cycle synchronization in cattle.
• Initiating these protocols with GnRH or estradiol benzoate (EB) may influence ovarian dynamics differently.
• Understanding these differences is key to optimizing artificial insemination protocols in Nelore cattle.

Purpose of the Study:

• To compare ovarian dynamics and progesterone levels in Nelore cattle using P4-based estrous synchronization protocols initiated with GnRH, EB, or no additional treatment.
• To evaluate the effects of different initiation treatments on follicle wave emergence and corpus luteum (CL) formation.
• To determine if distinct synchronization mechanisms impact final ovulation rates.

Main Methods:

• Three experiments involving Nelore cattle (cows and heifers) were conducted.
• Animals were assigned to P4-based protocols initiated with GnRH (G-7d), EB (EB-7d/EB-9d), or P4 only (P-5d).
• Daily ovarian ultrasonography and blood sampling were performed to monitor follicle dynamics, CL presence, and progesterone (P4) concentrations.

Main Results:

• The G-7d protocol resulted in higher ovulation rates after Day 0, with more CL and higher P4 levels at PGF treatment compared to EB and P-5d protocols.
• Both G-7d and EB protocols synchronized follicle wave emergence effectively (92.1%), but through distinct mechanisms (earlier emergence in G-7d, atresia followed by emergence in EB).
• The P-5d protocol showed limited synchronization of follicle wave emergence (30.0%), while final ovulation rates were similar across all treatments (85.7%).

Conclusions:

• GnRH initiation (G-7d) enhances early ovulation and progesterone profiles compared to EB and P-5d protocols.
• EB and GnRH protocols synchronize follicle dynamics via different mechanisms, impacting intermediate reproductive parameters but not final ovulation.
• All evaluated protocols show promise for Fixed-Time Artificial Insemination (FTAI) in Nelore cattle, though fertility requires further investigation.