Abstract

Artificial insemination, especially with the use of frozen semen, is one of the important tools for embryo transfer program in sheep. The present study investigated the effects of insemination times, breeds, and two inseminators on the fertility of ewes intrauterinally inseminated with frozen–thawed ram semen imported from New Zealand. At 8 sheep farms located in Hokkaido, Japan, during the breeding season (October to December) in 2005, a total of 64 mature (1- to 6-year old) Suffolk (32 heads) and Polled Dorset (32 heads) ewes were used. The ewes were treated with controlled intravaginal drug release (CIDR containing 0.3 g progesterone; Pharmacia & Upjohn, Ltd., Auckland, New Zealand) for 12 days and an injection of 500 IU equine chorionic gonadotropin one day before CIDR removal. The fixed-time intrauterine inseminations (early: 43–46 h; late: 47–50 h) after CIDR removal were performed using the frozen–thawed semen from a Suffolk and Polled Dorset ram by two inseminators. The effects of breeds (Suffolk and Polled Dorset), fixed-time insemination times (early and late phases), and two inseminators on pregnancy (number of pregnant ewes/number of ewes inseminated, 60 days after insemination) and lambing (number of lambed ewes/number of ewes inseminated) rates were analyzed by chi-square test. The prolificacy was compared by Student's t-test, and differences were also analyzed by Tukey's omega procedure. The effect of the different farms on fertility was not examined due to the small numbers of ewes per farm. Pregnancy (60.0 and 72.4%, respectively) and lambing (60.0 and 71.4%, respectively) rates were not significantly different between Suffolk and Polled Dorset ewes. The inseminators also did not affect pregnancy (62.6 and 68.8%) and lambing (62.6 and 67.7%) rates. For the insemination times, the lambing rate tended to be higher (P ≤ 0.10) in the early insemination than in the late insemination (76.7% and 53.6%, respectively). The present results show acceptable fertility in ewes inseminated with Suffolk and Polled Dorset frozen semen imported from New Zealand. The early intrauterine insemination (43–46 h after CIDR removal) tended to result in higher fertility than the late insemination (47–50 h after CIDR removal). From 38 lambed ewes, 60 newborn lambs were produced, and this has provided new blood lines of Suffolk and Polled Dorset sheep in Japan.