Effect of injection vehicle of progesterone on peripheral plasma progesterone levels and its relationship to fertility at controlled oestrus.

Abstract

The effect of vehicle of progesterone injection on peripheral plasma progesterone levels was examined in ewes. In Experiment 1 12 spayed ew es received im 10 or 20 mg progesterone in peanut oil or propylene glycol/day for 12 days. Blood samples were obtained for 4 days starting on the 10th day in the morning before injection of progesterone and 6 hours after injection. In Experiment 2 6 spayed ewes received a single injection of 10 mg progesterone in peanut oil or propylene glycol. Blood samples were obtained before injection and every 15 minutes up to 2 hours and periodically thereafter to 24 hours postinjection. Regardless of vehicle or dose in Experiment 1 progesterone levels 6 hours postinjection were higher than before injection. Injection in propylene glycol resulted in lower levels than did injection in peanut oil. There were no differences between vehicles during the 2 days after termination of injections. After 10 mg progesterone in propylene glycol (Experiment 2) the level was highest at 15 minutes (mean 1.6 ng/ml) and then declined steadily. With peanut oil there was a peak at 30 minutes (mean 1.58 ng/ml) with smaller peaks at 105 minutes (mean 1.46 ng/ml) and 6 hours (1 ng/ml). These results suggest that lower rates of egg re covery fertilization and lambing after control of estrus by injection of progesterone in propylene glycol are due to inadequate levels of progesterone before estrus.