Abstract

According to the importance of lambing rate to profitability of sheep holders, this trial investigated the effects of pregnant mare serum gonadotropin (PMSG) injection a day prior or at controlled intravaginal drug-releasing device (CIDR) removal on multiple births in synchronized Afshari ewes. 16 cycling, multiparous fat-tailed Iranian Afshari ewes, weighing 66.5±2.5 kg, were used in the trial. The ewes were randomly allocated in equal numbers (n = 8) to two treatment groups. The estrous cycles were synchronized using CIDR’s inserted for a period of 14 days. In a group (PMSG/ d -1) of ewes, PMSG (300 IU) intramuscularly (IM) was injected a day (day = -1) prior to CIDR removal (day 0 = day of CIDR removal) and another group (PMSG/ d 0) received PMSG at CIDR removal (day 0). 48 following CIDR removal (day 1), GnRH (50 µg) was administered to half of each group of ewes and ewes were mated with the proven rams. Ovarian follicular activity was monitored with the aid of transrectal ultrasonograpy: on the day of CIDR removal (day 0) and a day later (at estrus, day 1). Pregnancy was diagnosed with the aid of transabdominal ultrasonograpy 30 days after mating. Following PMSG (day = -1) treatment, the mean number of large follicles on both ovaries increased from CIDR removal to estrus (1.50±0.61 vs. 2.78±0.69) (P < 0.05). Lambing rate in PMSG/ d -1+GnRH, PMSG/ d -1 noGnRH, PMSG/ d0+GnRH and PMSG/ d0 noGnRH groups was 175, 125, 175 and 150% respectively. The results show that PMSG injection a day prior or at CIDR removal, caused large follicles development, but no significant difference was seen between the two treatment lambing rates and also, the lambing rate increased in group treated with GnRH.

Key words: Pregnant mare serum gonadotropin (PMSG), gonadotropin-releasing hormone (GnRH), Afshari ewes, lambing.