A novel therapeutic approach: The effect of letrozole-assisted synchronization on reproductive performance in suckling ewes during the non-breeding season.

The aim of the present study was to investigate the effect of letrozole treatment on the success of a progesterone-based synchronization protocol and fertility parameters in suckling Awassi ewes during the anoestrus period. One hundred and five multiparous, suckling Awassi ewes were used. Ewes (n = 105) were randomly divided into three groups as Control (n = 35), LET-4 (n = 35) and LET-0 (n = 35). Four days before the synchronization protocol (D-4), letrozole at a dose of 1 mg/kg was administered subcutaneously (s.c.) to the LET-4 group, while the LET-0 group received it on the day of protocol initiation (D0). The control group received 1 mL of benzyl alcohol s.c. at both time points as a placebo. Subsequently, intravaginal sponges containing 60 mg medroxyprogesterone acetate were inserted into all ewes (D0). The sponges were kept in the vagina for 7 days and 500 IU eCG and 75 μg d-cloprostenol were administered intramuscularly on the day the sponges were removed. Blood samples were taken for measurement of serum E2, FSH and P4 before sponge insertion (D-4 and D-2), on the day of sponge insertion (D0), after sponge insertion (D2 and D4), at mating (DM) and post-mating days (DPM-7, 14, 21, 35 and 50). Pregnancy was diagnosed by transrectal ultrasonography at DPM-21, 35, and 50. The overall mean serum FSH concentrations, calculated across all sampling days, were significantly higher in the LET-4 group (27.68 ± 1.56 mIU/mL) than in LET-0 (21.85 ± 0.87 mIU/mL) and Control (18.54 ± 0.91 mIU/mL) (P < 0.001). Subsequent to the FSH elevation, serum E2 levels at the time of mating were also significantly higher in LET-4 (22.24 ± 1.20 pg/mL) than in LET-0 (14.47 ± 0.92 pg/mL) and Control (10.77 ± 0.69 pg/mL) (P < 0.001). Post-mating P4 concentrations were likewise significantly elevated in the LET-4 group (3.79 ± 0.24 ng/mL) compared to LET-0 (3.29 ± 0.24 ng/mL) and Control (2.78 ± 0.24 ng/mL) groups (P < 0.001). Estrus rate was highest in the LET-4 group (90.62 %) compared to LET-0 (68.57 %) and Control (57.57 %) (P = 0.010). Pregnancy rate on day 50 was significantly greater in LET-4 (59.37 %) than in LET-0 (37.14 %) and Control (27.27 %) (P = 0.027). Fecundity rate was also highest in LET-4 (87.50 %) compared to LET-0 (57.14 %) and Control (33.33 %) (P < 0.001). No significant differences were observed among groups in litter size, twinning or pregnancy losses rates (P > 0.05). In conclusion, letrozole administration four days before the synchronization protocol significantly enhanced reproductive performance in suckling ewes during the non-breeding season. These results suggest that this approach could be a promising alternative strategy to improve protocol success without resorting to early weaning.