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

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.