Effect of GnRH administration at embryo transfer following a E2/P4 based protocol for FTET in Girolando heifers and cows

The aim of this study was to evaluate the effect of administering GnRH at the time of embryo transfer in a fixed time embryo transfer program (FTET) on the fertility of Girolando heifers, dry cows and lactating cows. At random days of the estrous cycle (D-11), 2mg estradiol benzoate (Gonadiol, MSD Animal Health, SP, Brazil) was administered IM and an intravaginal device with 1.9 g progesterone (P4, CIDR, Zoetis, SP, Brazil) was inserted. On D-4, 25 mg dinoprost (Lutalyse, Zoetis, SP, Brazil) was given IM. On D-2, 1.0 mg estradiol cypionate (ECP, Zoetis, SP, Brazil) was administered IM and the device was removed. On D18, the animals were examined by ultrasonography and only those with a CL received an embryo. On D-11, the animals were also classified according with their body condition score (BCS) following a 0 (very thin) to 5 (obese) point scale. At FTET, the animals were homogenously allocated to one of two groups: Control (no further treatment) or GnRH [100µg gonadorelin (Cystorelin, Merial, SP, Brazil)] IM. Two pregnancy diagnosis were conducted, the first on D31 and the second on D66. The analysis of binomial and continuous data were done using PROC GLIMMIX and MIXED of SAS, respectively. Significance was considered when P < 0.05 and tendency when 0.05 < P < 0.10. No interaction of treatment and animal category was observed on conception rate at 31 days (P = 0.33), 66 days (P = 0.32) and on pregnancy loss (P = 0.92). No effect of treatment was observed on 31 days [Control = 52.8% (477/903) vs. GnRH = 52.3% (473/904); P = 0.83] and 66 days conception rate [Control = 42.3% (382/903) vs. GnRH = 44.03% (398/904); P = 0.46]. However, treatment with GnRH tended to reduce pregnancy loss between 31 and 66 days [Control = 19.9% (95/477) vs. GnRH = 15.9% (75/473); P = 0.10]. Heifers had greater 66 days conception rate [49.5% (412/832)] than dry cows [37.0% (44/119); P < 0.01] and lactating cows [39.6% (610/1539); P < 0.01]. Lactating cows had greater pregnancy loss [21.4% (166/776); P < 0.01] compared to heifers [14.0% (67/479); P = 0.10] and tended to have greater pregnancy loss compared to dry cows [12.0% (6/50); P = 0.10). Effect of BCS on 66 days conception rate was observed in lactating cows from the Control group [Thin (BCS ≤ 2.5) = 26.8% (22/84) vs. Good (BCS ≥ 2.75) = 42.3% (203/481); P < 0.01]. Cows treated with GnRH at FTET had no effect of BCS on 66 days conception rate [Thin = 36.8% (33/89) vs. Good = 39.6% (184/468); P = 0.62]. In conclusion, the administration of GnRH at FTET may reduce pregnancy loss between 31 and 66 days of gestation and increase conception rate in lactating cows with low BCS.