109 COMPARISON OF METHODS FOR SYNCHRONIZING RECIPIENTS OF IN VITRO PRODUCED EMBRYOS

Abstract

The present study compared fertility, as pregnancy per embryo transfer (P/ET), and efficiency of recipient utilisation, as pregnancy per treated potential recipient (P/TX), in heifers receiving in vitro-produced embryos using synchronized oestrus after prostaglandin F2α (OESTRUS) or synchronized ovulation and fixed timed embryo transfer (FTET) with new or reused CIDR. In Expt. 1, heifers (n = 520) were randomly assigned to 1 of 3 groups: OESTRUS, FTET with new CIDR, or FTET with second-use CIDR (previously used for 5 days). Heifers in OESTRUS group (n = 166) were synchronized with two prostaglandin F2α 14 days apart and detection of oestrus performed using tail chalk during 5 days after the second prostaglandin F2α. Heifers in FTET were synchronized with a new CIDR (n = 178) or second-use CIDR (n = 176) using a modified 5-day CIDR-Synch; Day –8: CIDR inserted; Day –3: CIDR removed, prostaglandin F2α; Day –2: second prostaglandin F2α; Day 0: gonadotropin-releasing hormone to induce ovulation. In Expt. 2, heifers (n = 422) were randomly assigned to 1 of 2 groups: FTET with new CIDR or FTET with third-use CIDR (previously used twice for 5 days each time) using the FTET protocol described for Expt. 1. Fresh in vitro-produced embryos were transferred between 6 and 8 days after OESTRUS or gonadotropin-releasing hormone (FTET). All heifers were evaluated by transrectal ultrasonography on Day 32 and 60 for pregnancy detection. Measurements of P/ET and P/TX for both experiments were analysed by logistic regression (LOGISTIC procedure, SAS 9.4) using biologically meaningful covariates such as embryo stage and quality, interval from oestrus or GnRH to transfer, and technician in the statistical analyses. In Expt. 1, two preplanned contrasts were performed to compare differences between OESTRUS v. FTET, and between FTET with new v. second-use CIDR. The P/ET at Day 32 was similar (P = 0.50) with 41.3% (45/109) for OESTRUS and 43.4% (134/309) for FTET groups. Similarly, P/ET on Day 60 was 30.3% (33/109) for OESTRUS and 32.4% (100/309) for FTET groups (P = 0.37). However, P/TX heifer on Day 60 was greater (P = 0.04) in the FTET (28.2%; 100/354) compared to OESTRUS (19.9%; 33/166). This difference is attributed to a greater (P < 0.001) utilisation efficiency (transferred/treated) of heifers in FTET (87.3%) v. OESTRUS (65.6%). In the second contrast, P/ET on Day 32 were similar (P = 0.87) for FTET heifers synchronized with a new CIDR (43.9%, 69/157) v. second-use CIDR (42.8%, 65/152). In addition, P/TX heifer on Day 60 was also similar (P = 0.52) for heifers receiving a new or second-use CIDR (29.8%, 53/178 v. 26.7% 47/176). In Expt. 2, P/ET on Day 32 was similar (P = 0.73) for FTET with a new CIDR (41.0%, 77/188) or third-use CIDR (42.3%, 83/196). The P/TX heifer on Day 60 was also not different (P = 0.58) for new CIDR (28.6%, 61/213) v. third-use CIDR (31.1%, 65/209). Thus, use of FTET with new or used CIDR can produce similar P/ET and greater efficiency of recipient utilisation compared to OESTRUS.