Dose-response relationships of exogenous progesterone shortly after ovulation on estrous cycle length, blastocyst development and fertility in sheep

Maternal recognition of pregnancy is a period of competing signals; a luteolytic signal from the endometrium and an antiluteolytic signal from the blastocyst (s). Exogenous progesterone indirectly enhances these signals, causing premature release of prostaglandin and stimulating growth of blastocysts. In Experiment 1, 78 ewes were injected with vehicle or varying dosages of progesterone on Days 2–4 (Day 0 represents onset of estrus) for the purpose of comparing the minimal dose that shortened the estrous cycle, in non-pregnant ewes, to the minimal dose that enhanced growth of blastocysts to Day 13, in pregnant ewes. In Experiment 2, a field trial was conducted using Targhee and Polypay ewes to evaluate their fertility after treatment with vehicle or 6 mg of progesterone (n = 55). Analysis of plasma samples indicated that none of the dosages of exogenous progesterone altered concentrations of progesterone by Days 5 or 10, suggesting that treatment with exogenous progesterone failed to alter luteal function to Day 10. The minimal dose of progesterone that shortened (P<0.05) the estrous cycle was 6 mg and was the same dose that began to stimulate (P<0.05) blastocyst growth. Lambing rates of Targhee ewes were not different following treatment with exogenous progesterone. However, the lambing rate of Polypay ewes increased (P<0.05) from 200 to 256%. These data suggest that treatment of sheep, predisposed to an ovulation rate greater than two, with progesterone improved embryonic survival by a still unknown mechanism(s) that might have also advanced luteolytic and antiluteolytic signals.