Estradiol treatment and double dose of GnRH for resynchronization protocols and timed artificial insemination in lactating dairy cows

Abstract

The aim of this study was to compare pregnancy per AI (P/AI) in resynchronization (Resynch) protocols using different hormonal combinations (estradiol benzoate [EB], a double dose of GnRH, or EB with a double dose of GnRH) administered on the day of progesterone (P4) device insertion. We hypothesized that combined administration of EB and GnRH on Day 25 after the first timed AI (TAI) is more efficacious than the administration of EB or GnRH alone in the resynchronization of estrus and ovulation in lactating dairy cows. The secondary objective was to compare P/AI in cows treated with two different GnRH agonists (gonadorelin acetate or buserelin acetate) on the day of P4 device insertion. This study included lactating Holstein cows (382 in their first lactation and 629 in their second or more lactations), averaging 135.4 ± 15.6 days in milk, with a body condition score of 2.8 ± 0.1, and producing 41.3 ± 0.5 kg of milk per day. On Day 25 post first timed AI (TAI), all cows received a previously used CIDR-B device (1.9 g P4) and were randomly assigned to receive one of the following treatments: 2-mg EB, a double dose of GnRH, or a combination of 2-mg EB and a double dose of GnRH. Cows receiving GnRH were randomly divided into the 200-μg gonadorelin acetate group or 20-μg buserelin acetate group. On Day 32, all cows were examined via ultrasonography, and those that were not pregnant received 500 μg of cloprostenol (PGF2α). On Day 33, the CIDR-B devices were removed, and all cows received a second dose of PGF2α, 1 mg of estradiol cypionate (ECP), and 400 IU eCG, with all cows receiving tail paint for estrus detection. Cows that demonstrated estrus at 48 h were inseminated at that time, whereas those that did not received 100 μg of gonadorelin and were inseminated at 56 h. Cows in the EB + double-dose GnRH group exhibited greater estrus expression (93.6 %, P < 0.05) than those in the EB (88.1 %) or double-dose GnRH (92.0 %) group, but no difference in the P/AI was observed among the groups (EB = 44.1 %, double-dose GnRH = 49.0 %, EB + double-dose GnRH = 49.0 %; P = 0.94). Similarly, no significant difference in the P/AI was observed between cows treated with gonadorelin acetate (50.0 %) and those treated with buserelin acetate (48.1 %, P = 0.31). In conclusion, the combined administration of EB and a double dose of GnRH does not improve P/AI compared with the administration of either hormone alone. Furthermore, both agonists produced comparable P/AI, confirming that they are equally effective when administered during P4 device insertion.