Five-day resynchronization programs in dairy cows including presynchronization and progesterone at two stages following artificial insemination1

Two experiments were conducted to assess pregnancy outcomes after a 5-d Ovsynch-56 Resynch program [RES; GnRH injection (d 0) 5 d before and 56 h after the first of 2 PG injections on d 5 and 6, and timed artificial insemination (TAI) on d 8] with or without a 5-d progesterone-releasing intravaginal controlled internal drug release (CIDR) insert applied at d 0. In experiment 1, nonpregnant cows were enrolled on d 35 post-AI: d-35 RES-CON (n = 454) or d-35 RES-CIDR (n = 495). Blood was collected on d 0 for progesterone assay. Only 76.4% of enrolled cows had high (≥1 ng/mL) progesterone on d-35 nonpregnant diagnoses (NPD). Pregnancy per AI (P/AI) diagnosed by transrectal palpation at 35 and 63 d post-TAI did not differ between treatments. An interaction between treatment and progesterone status on d 35 was detected. The d-35 RES-CIDR cows with progesterone ≥1 ng/mL at treatment initiation had reduced P/AI compared with d-35 RES-CON (27.7 vs. 36.4%), whereas the opposite effect occurred for d-35 RES-CIDR cows with progesterone <1 ng/mL compared with d-35 RES-CON (39.5 vs. 31.7%). In experiment 2, cows were enrolled on d 31 post-AI (NPD): (1) d-31 PG-3-G-RES (n = 208): Pre-PG on d 31, Pre-GnRH on d 34, and RES on d 41; (2) d-41 RES-CON (n = 208); or (3) d-41 RES-CIDR (n = 208) as in experiment 1, but starting on d 41. In a subset of cows (more than 102 per treatment), blood samples were collected for progesterone assay and ovarian structures were mapped by ultrasonography on d 31, 34, 41, 46, 48, and 55. Pregnancy was diagnosed by ultrasonography on d 31 and 78 post-TAI. The proportion of cows with progesterone ≥1 ng/mL on d 31 was 70.6%. More d-31 PG-3-G-RES cows ovulated after Pre-GnRH (60.4%) than did d-41 RES-CON (16.5%) or d-41 RES-CIDR (9.3%) cows. More d-31 PG-3-G-RES cows had luteolysis after Pre-PG on d 31 (75.1%) than did d-41 RES-CON (7.5%) and d-41 RES-CIDR (8.6%) cows. Although more d-31 PG-3-G-RES cows were inseminated earlier, P/AI did not differ among treatments for cows inseminated after estrus or by TAI. Pregnancy per AI, however, was greater in cows starting RES on d 41 when progesterone was ≥1 ng/mL compared with <1 ng/mL (44.8 vs. 31.6%), respectively. We conclude that presynchronization of estrous cycles in nonpregnant cows with PG and GnRH before a d-41 RES 5-d program does not improve P/AI. Furthermore, addition of the CIDR insert in a 5-d program decreased P/AI only when treatment was initiated in cows on d 35 post-AI and when progesterone was ≥1 ng/mL (no corpus luteum) but not when beginning treatment on d 41.