Vol. 9, Issue 8, Part N (2025)

This study assessed the efficacy of varying doses of letrozole-impregnated intravaginal sponges for estrus synchronization in ewes, focusing on sponge retention, estrus response, interval to estrus, duration of estrus, and conception rate. Thirty-six healthy, cyclic, non-pregnant ewes were randomly assigned to six groups (n=6). Four groups received polyurethane sponges containing 2.5, 5, 7.5, or 10 mg letrozole; the placebo and positive control groups received blank sponges and Avikesil-S^® sponges, respectively. On Day 7, all ewes received 125 µg PGF₂α intramuscularly at sponge removal and were observed for estrus expression over 72 h. Natural mating followed, and conception was confirmed by ultrasonography 30 days post-mating. Sponge retention was 100 per cent across all groups, indicating excellent device stability. Estrus response was highest (100%) in the 7.5 mg letrozole and Avikesil-S^® groups, followed by 5 mg (83.33%), 2.5 mg (66.67%), and 50 per cent in both 10 mg and placebo groups. The interval from sponge removal to estrus increased significantly (p<0.05) with letrozole dose, from 49.00±1.29 h (2.5 mg) to 81.33±0.67 h (10 mg), compared with 37.00±0.86 h (Avikesil-S^®) and 38.33±1.20 h (placebo). Estrus duration also extended with dose, from 27.00±1.29 h (2.5 mg) to 52.00±1.15 h (10 mg), compared with 32.00±1.15 h (Avikesil-S^®) and 28.67±1.76 h (placebo). Conception rates were highest in the 7.5 mg letrozole and Avikesil-S^® groups (83.33%), followed by 5 mg (80.00%), 2.5 mg (75.00%), and 66.67 per cent (10 mg and placebo), with no significant differences (p>0.05). These findings indicate that a 7.5 mg intravaginal letrozole sponge provides optimal estrus synchronization and fertility outcomes, comparable to a conventional progestogen-based protocol, offering a promising non-steroidal alternative for reproductive management in ewes.