The Effect of Post-Mating Progesterone Supplement on Pregnancy and Lambing Rates of Ewes Bred Out-of-season

Abstract

The objective of this experiment was to determine whether post-mating progesterone (P₄) supplement improves pregnancy and lambing rates in ewes bred during the seasonal anestrous period. In June, 39 synchronized to estrus Awassi ewes were allowed with four harnessed fertile rams immediately following CIDR-G device removal (day 0 and 0 hour). Five days following ram introduction, ewes were randomly assigned to four groups to be treated post-mating with intramuscular injections of 20 mg P₄ supplement once daily from day 5 to day 9 (P₄-D5-9), days 10 to 14 (P₄-D10-14), days 5 to 14 (P₄-D5-14) or did not receive P₄ supplement (control). Blood samples were collected from all ewes for P₄ analysis. Progesterone concentrations prior to CIDR-G insertion were basal and no differences in P₄ concentrations were found on days -12, -10 and between days 0 and 5 among groups. Progesterone concentrations between days 5 and 15 differed (p<0.001) significantly due to treatment effect. Maximum P₄ concentrations were reached on day 9 in group P4-D5-9 and between days 11 and 15 in groups P₄-D10-14, P₄-D5-14, and control. Pregnancy was diagnosed based on day 19 P₄ levels and day 30 ultrasonic examination in 5/10, 6/10, 5/10, and 5/9 ewes in groups P₄-D5-9, P₄-D10-14, P4-D5-14 and control, respectively. Pregnancy loss was detected by ultrasonography on day 45 in 3/6 and 3/5 ewes in P₄-D10-14, P₄-D5-14 groups only. Overall pregnancy (53.8%) and lambing (41%) rates were similar among groups and were not influenced by P₄ supplement. In conclusion, P₄ supplement administered intramuscularly between days 5 and 14 post-mating is not effective in improving pregnancy, embryonic survival and lambing rates in Awassi ewes pretreated out-of-season. Pregnancy loss which occurred only in P₄-D10-14 and P₄-D5-14 groups maybe attributed to factors including the sharp decrease in P₄ concentrations on day 15 and stress experienced during the period of maternal recognition of pregnancy process.