Effects of Age Category and eCG Dose on Reproductive Performance in Ile-de-France Ewe Lambs and Adult Ewes Under Seasonal-Transition Field Conditions

This field study evaluated the effects of 400 and 500 IU equine chorionic gonadotropin (eCG) on oestrus response, fertility, and lambing performance in Ile-de-France ewe lambs and adult ewes under seasonal-transition field conditions. A total of 914 animals were assigned to four groups according to age category and eCG dose: ewe lambs receiving 400 IU (EL400, n = 173) or 500 IU eCG (EL500, n = 169), and adult ewes receiving 400 IU (AE400, n = 301) or 500 IU eCG (AE500, n = 271), following a 7-day intravaginal progesterone-based synchronization protocol. Oestrus rate did not differ among groups (p = 0.861), indicating that both doses were similarly effective in inducing behavioural oestrus, whereas the interval to oestrus onset showed significant main effects of both age category (p < 0.001) and eCG dose (p < 0.001), with no significant age × dose interaction (p = 0.559). Using a 2 × 2 factorial analysis, significant main effects of age category were observed for outcomes including pregnancy rate following oestrus, overall pregnancy rate, and fecundity (p = 0.001, p = 0.009, and p = 0.004, respectively), with higher values in adult ewes than in ewe lambs regardless of eCG dose. In contrast, 500 IU eCG was associated with a greater overall litter size and multiple birth rate than 400 IU eCG (p = 0.039 and p = 0.033, respectively). The age × dose interactions for litter size and multiple birth rate were not statistically significant (p = 0.058 and p = 0.071, respectively). The age × dose interaction for lambing rate was also not statistically significant (p = 0.064). These results indicate that age category and eCG dose were associated with different aspects of reproductive performance. Under the specific seasonal-transition field conditions of this single-breed, single-farm study, selecting the eCG dose according to the reproductive objective may represent a practical management consideration.