| The reproductive cycles represent continued opportunities for the cow to become pregnant, and are determined by physiological events such as growth of follicular waves, development of an ovulatory follicle, ovulation, and corpus luteum (CL) development and CL regression (luteolysis). Complete luteolysis is necessary to have optimal conditions for fertilization, whereas during early pregnancy the embryo prevents luteolysis by release of interferon-tau (INFT) and prostaglandins E. After successful maintenance of the pregnancy past the critical period there are a number of pregnancies that fail, indicating the existence of additional periods which are susceptible to pregnancy loss and/or luteolysis that have not been explored. Both luteolysis and prevention of luteolysis involve local pathways of communication between the uterus and the ovary. This thesis studies the utero-ovarian relationships during the period of luteal regression preceding the insemination, the period of maternal recognition of pregnancy, and the period of potential pregnancy loss thereafter. We demonstrated that the size of the uterus influences the fertility outcome, with differences in pregnancies per artificial insemination between extreme quartiles of 21% for primiparous (61% vs 40%) and 23% for multiparous cows (49% vs 26%). Cows with larger uteri are associated with decreased P/AI, regardless of the parity. This association is probably related to the ability of the embryo to produce INFT and avoid luteolysis during the critical period. By using a model of induction of contralateral accessory CL, we demonstrated a novel pattern of CL regression during a later time after the critical period that peaks between Days 41 to 47, and decreases afterwards, suggesting additional mechanisms that prevent luteolysis after this first inhibition of luteolysis by embryonic INFT. We suggest that adequate development of placental attachment, associated with increased blood flow, reduces the amount of PGF reaching the ovary adjacent to the pregnancy. Finally, the use of a second dose of PGF at the end of synchronization with Double Ovsynch protocol, increases the percentage of cows obtaining complete luteolysis from 83% to 97% supporting the hypothesis that improvements in achieving complete luteolysis by using this strategy could have potential beneficial effects on fertility in dairy cattle. |
