DOSE-RESPONSE RELATIONSHIPS OF EXOGENOUS PROGESTERONE SHORTLY AFTER OVULATION ON ESTROUS-CYCLE LENGTH, BLASTOCYST DEVELOPMENT AND FERTILITYIN SHEEP

Maternal recognition of pregnancy is a period of competing signals; a luteolytic signal from the endometrium and an antiluteolytic signal fr om the blastocyst(s). Exogenous progesterone indirectly enhances these signals, causing premature release of prostaglandin and stimulating g rowth of blastocysts. In Experiment 1, 78 ewes were injected with vehi cle or varying dosages of progesterone on Days 2-4 (Day 0 represents o nset of estrus) for the purpose of comparing the minimal dose that sho rtened the estrous cycle, in non-pregnant ewes, to the minimal dose th at enhanced growth of blastocysts to Day 13, in pregnant ewes. In Expe riment 2, a field trial was conducted using Targhee and Polypay ewes t o evaluate their fertility after treatment with vehicle or 6 mg of pro gesterone (n=55). Analysis of plasma samples indicated that none of th e dosages of exogenous progesterone altered concentrations of progeste rone by Days 5 or 10, suggesting that treatment with exogenous progest erone failed to alter luteal function to Day 10. The minimal dose of p rogesterone that shortened (P < 0.05) the estrous cycle was 6 mg and w as 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 treatm ent 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 .