Effect of the breed, reproductive season, doses and time of application of eCG in the estrous cycle in hair ewe lambs

Abstract

Objective: to evaluate the effect of breed, reproductive season, dose and time of application of equine chorionic gonadotropin (eCG) on the estrous cycle and ovarian activity in hair ewe lambs. Design/methodology/approach: 216 Hair lambs were used (62 Dorper, 69 Katahdin y 85 Pelibuey), 91 in the high reproductive season and 125 in the low reproductive season, they were synchronized with intravaginal sponges containing 20 mg of fluorogestone acetate (FGA) and equine chorionic gonadotropin (eCG; 200 and 300 IU) intramuscularly. The treatments are the breed, reproductive season, dose and time of application of eCG. The presence of estrus was analyzed under a logistic regression model, the interval to estrus and ovulation rate were analyzed by analysis of variance using a completely randomized design with a 2x2x2x3 factorial arrangement with the PROC LOGISTIC and PROC GLM procedures. Results: the breed influenced (P<0.01) the presence of estrus, Dorper ewe lambs presented 9.74 times more possibilities than the Pelibuey. The interval to estrus was shorter in duration (P<0.05) in Dorper (29.5±0.9 h) and Katahdin (29.1±0.9 h) compared to Pelibuey (34.8±0.9 h). The interval to estrus was lower (P<0.05) with the application of 200 or 300 IU of eCG 24 h before the end of the protocol, compared to the application of 200 IU of eCG at the time of progestogen withdrawal. Ovulation rate was only affected by breed (P<0.05), being higher in Pelibuey (2.4±0.1) compared to Dorper (2.0±0.1) and Katahdin (1.9±0.1). Study limitations/implications: To complement the study, it would be convenient to take the research to a second phase where the gestation part of the females is included to relate it to the ovulation rate, and also to make measurements of ovarian structures by means ultrasound. Findings/conclusions: Estrus and ovarian activity in hair ewe lambs synchronized with progestogens is mainly influenced by breed.