105 Optimization of a five-day fixed-time embryo transfer program in dairy heifers: Use of gonadotrophin-releasing hormone at initiation of the protocol
Authors: Sala R. , Carrenho-Sala L. , Absalon-Medina V. , Lopez A. , Fosado M. , Moreno J. , Wiltbank M. , Garcia-Guerra A. 
Published: December 2019

Optimized fixed-time embryo transfer (FTET) protocols for synchronization of recipients have the potential to improve the overall efficiency and profitability of embryo transfer (ET) programs. The objective of the present study was to evaluate the effect of dose of gonadotrophin-releasing hormone (GnRH) at initiation of a 5-day synchronization protocol for FTET. Holstein heifers (n=2689) at two locations were synchronized using a 5-day CO-Synch protocol as follows: Day 0: CIDR inserted, Day 5: CIDR removed, prostaglandin (PG)F2α treatment (500μg cloprostenol), Day 6: PGF2α treatment, Day 8: GnRH (100μg of gonadorelin). On Day 0, at the time of CIDR insertion, heifers were assigned in a completely randomised design to the following groups: Single (a single dose of GnRH; 100μg of gonadorelin), Double (200μg of gonadorelin) or No GnRH (control). All heifers received an Estrotect patch placed on Day 5 and evaluated for signs of oestrus on Day 8. At location A, heifers were evaluated by ultrasonography 5 days after GnRH to determine presence and size of corpus luteum (CL), whereas at location B presence and location of CL were determined by transrectal palpation at the time of transfer. Heifers with a CL received an embryo 7±1 days after GnRH administration, and pregnancy was determined by ultrasonography 41 and 63 days after GnRH. Data were analysed by generalized linear mixed models. Oestrus expression was greater in heifers that received Single and Double GnRH than in the No GnRH group (P=0.001). Similarly, utilisation rate (number transferred per number treated) was greater for heifers in the Single and Double GnRH group than for those in the No GnRH group (P=0.02). Pregnancy data were analysed for a subset of recipients using data from Day 41 (n=2267) and Day 63 (n=2042). The analysis of fertility outcomes included as covariates the type of embryo (invitro fresh or frozen and invivo fresh or frozen), embryo stage, embryo quality, interval from GnRH to transfer, and oestrus expression. Pregnancies per embryo transfer (P/ET) at Days 41 and 63 were not different between treatment groups (P=0.86), and there was no interaction between type of embryo and treatment (P&gt;0.15). Pregnancy loss between Days 41 and 63 was not different (P=0.49) between treatments groups. In conclusion, the removal of the initial GnRH from a 5-day FTET protocol resulted in a slight but significant reduction in the utilisation rate and the percentage of heifers showing oestrus. However, there was no detrimental effect on fertility. As a result, the overall cost of the FTET program can be reduced by eliminating the need for the initial GnRH treatment without compromising fertility.

Table 1.Reproductive performance in recipients receiving different doses of gonadotrophin-releasing hormone (GnRH) at initiation of the synchronization protocol

Treatment
Oestrus (n)
Utilisation rate (n)
P/ET1 D41 (n)
P/ET D63 (n)
Pregnancy loss (n)

No GnRH
69.2%B (621/898)
85.0%B (763/898)
41.6% (308/740)
39.9% (268/672)
4.3% (12/280)

Single GnRH
76.1%A (685/900)
88.8%A (799/900)
42.7% (329/770)
39.5% (272/689)
6.5% (19/291)

Double GnRH
75.3%A (671/891)
88.7%A (790/891)
41.5% (314/757)
38.9% (265/681)
5.4% (15/280)

A,BValues with different superscripts within a column differ (P&lt;0.05).
1P/ET=pregnancies per embryo transfer.