Development of a letrozole-based synchronization protocol for fixed-time artificial insemination in beef cattle

Area of Science:

• Veterinary Medicine
• Animal Reproduction
• Endocrinology

Background:

• Synchronization of ovulation is crucial for efficient artificial insemination in cattle.
• Letrozole has emerged as an alternative to progesterone for estrous cycle manipulation.
• Intravaginal letrozole-releasing devices (LRDs) offer a novel approach to drug delivery for estrous synchronization.

Purpose of the Study:

• To evaluate ovulation synchrony after a 4-day letrozole treatment.
• To compare the efficacy of letrozole-based protocols with existing synchronization methods for fixed-time artificial insemination (FTAI).
• To test a new intravaginal letrozole-releasing device (LRD) with and without pre-synchronization.

Main Methods:

• Experiment 1: Cattle received an LRD for 4 days, followed by PGF and GnRH at varying intervals.
• Experiment 2: Heifers and cows were assigned to LRD, estradiol+progesterone, or 5-d Co-synch+PRID protocols for FTAI.
• Experiment 3: Heifers were pre-synchronized or not before receiving either an X-LRD or T-LRD for 3 days, followed by FTAI.

Main Results:

• A shorter interval to GnRH after PGF improved ovulation synchrony in cows (Exp 1).
• The LRD protocol resulted in lower pregnancy per artificial insemination (P/AI) compared to other protocols (Exp 2).
• The X-LRD enhanced plasma letrozole concentrations, but pre-synchronization and LRD type did not affect pregnancy rates (Exp 3).

Conclusions:

• While LRDs can achieve ovulation synchrony, the protocol may require optimization to improve pregnancy rates.
• The new X-LRD demonstrates improved drug delivery, warranting further investigation for estrous synchronization efficacy.
• Interval between LRD removal and GnRH/FTAI may need adjustment for optimal outcomes.