Development and evaluation of an injectable slow‐release progesterone formulation for estrus synchronization in ewes out of the breeding season

This study was aimed at developing a type of slow‐release progesterone micro‐particles useable in a single intramuscular injection for estrus synchronization in non‐breeding season ewes. A total of 66 ewes were randomly assigned into four groups: CIDR (n = 16): exposed to intravaginal CIDR for 12 days, and three experimental groups, i.e., T100 (n = 16), T150 (n = 17) and T200 (n = 17), receiving a single intramuscular injection of 100, 150 and 200 mg slow‐release progesterone, respectively. Blood sampling was performed on all ewes at five different times, and the ELISA method measured progesterone levels. No significant differences were observed in progesterone levels among the groups in each sampling time. More than 90% of ewes in the CIDR, T100 and T150 groups and all those in T200 showed estrus behaviour, and the rate was not significantly different between groups. The difference in the mean interval from progesterone treatment to estrus was also insignificant. The parturition rate declined by increasing the dose of injected progesterone; although it was similar in CIDR and T100 groups, it decreased significantly in T150 and T200. Since our injectable progesterone formulation was successful in the induction and synchronization of estrus in ewes out of the breeding season, it can be applied as an alternative to the conventional progesterone containing intravaginal devices.