Abstract The aim was to examine the exogenous supply of progesterone, and the influence of the corpus luteum (CL) at the beginning of the estrus synchronization protocols in sheep, for 6 and 11 days, on reproductive variables. The sheep were presynchronized to obtain a premature or functional CL at the beginning of the synchronization period to be evaluated. Estrus presentation was lower in short treatments; This triggered a difference (p < 0.05) in pregnancy rate between short and conventional treatments. On the day of estrus, steroid concentration was different: progesterone was higher in short treatments; while estradiol was lower in the short treatment with CL. The presence of a premature or functional CL at the initiation of the treatments commonly used to synchronize estrus does not modify the expression of the reproductive variables; Nevertheless, in short treatments the expression is affected. It is important to use ultrasonography to corroborate what happened at the ovarian level.

Resumen El objetivo fue evaluar el aporte exógeno de progesterona, y la influencia del cuerpo lúteo (CL) al iniciar protocolos de sincronización de estros en ovejas, por 6 y 11 días, sobre las variables reproductivas. Las ovejas fueron presincronizadas para obtener un CL inmaduro o funcional al iniciar el periodo de sincronización a evaluar. La presentación de estros fue baja en tratamientos cortos; esto desencadenó discrepancias (p < 0.05) en porcentaje de gestación de tratamientos de 6 días y los tradicionales de 11 días. El día del estro la concentración de esteroides fue diferente, progesterona fue mayor en tratamientos cortos; mientras que estradiol fue menor en el tratamiento corto con CL. La presencia de un CL inmaduro o funcional al iniciar tratamientos para sincronizar estro convencionales no modifica la expresión de las variables reproductivas; sin embargo, en tratamientos cortos la expresión se ve afectada. Es importante el uso de ultrasonografía para corroborar lo que pasa a nivel ovárico.

ABSTRACT The objective of the present research was to determine the effect of long synchronization protocols based on reused progesterone devices (controlled internal drug release [CIDR]) associated with different doses of equine chorionic gonadotropin (eCG) on reproductive variables in crossbred sheep (Suffolk × Kathadin × Dorset). The CIDR were used for eleven days in a previous study in sheep from the same herd and were washed and disinfected before reusing. Sixty-four sheep, in the reproductive season, were randomly assigned to four experimental groups (n = 16). Treatments consisted of a group with 10 d CIDR and 300 IU eCG; a group with 10 d CIDR and 400 IU of eCG; a group with 12 d CIDR and 300 IU of eCG; and a group with 12 d CIDR and 400 IU of eCG. A completely randomized design was used. There was an estrous presentation rate of 100% in all treatments. The begining of estrous, gestation rate, fertility rate, type of parturition, and prolificacy index were equal between groups. Progesterone serum concentration was higher in sheep from the 10 d CIDR groups. The CIDR, reused for the second time, associated with 300 or 400 IU of eCG for estrous synchronization in sheep, are effective to obtain good pregnancy rates and ensures higher prolificacy rates.