114 Effect of length of progesterone device administration during a modified CO-Synch for fixed-time embryo transfer in heifers

Extending the length of progesterone (P4) treatment during CO-Synch can potentially increase preovulatory follicle and corpus luteum (CL) size, which in turn can affect fertility after fixed-time embryo transfer (FTET). Thus, the aim of the present study was to evaluate the effect of length of P4 device administration on the reproductive performance of heifers submitted to FTET using a modified CO-Synch. A total of 780 heifers at 17.2 ± 2 months of age, with moderate body condition score (3.4 ± 0.4; scale 1–5) and arranged in 12 weekly cohorts were randomly assigned to a 5-day or 6-day modified CO-Synch program for FTET. Heifers received a new, second-use or third-use intravaginal P4 device (CIDR, 1.38 g of P4, Zoetis) on Day −9 or Day −8, for the 6-day and 5-day group, respectively. On Day −3 all heifers had the P4 device removed, 500 μg of cloprostenol sodium (PGF, Parnell) was administered and an Estrotect patch applied. Seventy-two hours after P4 device removal (D0) 100 µg of gonadorelin acetate (GnRH, Parnell) was administered to all heifers and Estrotect patches evaluated for signs of oestrus. Ultrasonography was performed on Day 6 to determine presence and size of the CL. Heifers with a CL received a 6.5-day-old IVF embryo 6.5 days after GnRH administration and pregnancy was determined using ultrasonography 32 and 60 days after GnRH. Data were analysed using generalized linear mixed models and results are presented in Table 1. There was no effect (P > 0.10) of P4 device reutilisation nor a treatment by P4 device interaction for any of the outcomes evaluated. The percentage of heifers that expressed oestrus was not different between treatment groups (P = 0.83). Similarly, utilisation rate (transferred/treated) was not different (P = 0.42) for heifers in the 5-day and 6-day group. There was no difference in CL volume between heifers administered a 5-day (4807 ± 135 mm³) or 6-day (4928 ± 135 mm³) modified CO-Synch. The analysis of fertility outcomes included embryo stage, embryo quality and interval from GnRH to transfer as covariates. Pregnancies per embryo transfer (P/ET) at Day 32 and Day 60 were not different between treatment groups (P > 0.6). In addition, there was no effect (P = 0.44) of length of P4 administration on pregnancy loss between Day 32 and Day 60. In conclusion, length of P4 administration during a modified CO-Synch protocol did not affect expression of oestrus, utilisation efficiency or fertility after FTET in heifers.