Effect of day of the estrous cycle at the initiation of a timed artificialinsemination protocol on reproductive responses in dairy heifers

Our objectives were to identify stages of the estrous cycle at which initia tion of a timed artificial insemination (Ovsynch/TAI) protocol may reduce p regnancy rates and to monitor ovarian follicle dynamics and corpus luteum d evelopment after initiation of the Ovsynch/TAI protocol at different stages of the cycle. Cycling Holstein heifers (n = 24) were injected twice nifh p rostaglandin F-2 alpha to induce estrus and were scanned by ovarian ultraso nography to determine the day of ovulation (d 0). Heifers were assigned to initiate the Ovsynch/TAI protocol at d 2 (n = 5, 5 (n = 5), 10 (n = 4), 15 (n = 5), or 18 (n = 5) of the cycle. The Ovsynch/TAI was initiated with an injection of gonadotropin-releasing hormone agonist followed 7 d later with an injection of prostaglandin F-2 alpha. At 36 h after injection of prosta glandin F-2 alpha, heifers were injected with gonadotropin-releasing hormon e agonist and inseminated 16 h later. Heifers were scanned daily during the Ovsynch/ TAI protocol and every other day after insemination until 16 d la ter. Blood samples were collected daily starting at the Ist day heifers wer e scanned and continued until 16 d after insemination. Initiation of the Ov synch/TAI protocol at d 15 of the estrous cycle caused heifers to ovulate p rior to insemination. A shortened return to estrus (< 16 d) was caused by o vulation failure to the second gonadotropin-releasing hormone injection, by incomplete regression of the corpus luteum, and by short life-span of the induced corpus luteum. Day of the cycle in which the Ovsynch/TAI protocol i s initiated affects dynamics of follicular development, plasma progesterone profiles, and occurrence of premature ovulation. Size of the pre-ovulatory follicle was associated positively with subsequent progesterone concentrat ions following insemination.