Changing the route of GnRH administration according to the initial cyclic phase of the Ovsynch protocol may optimize reproductive outcomes in dairy cows.

Abstract

BACKGROUND: This study aimed to investigate the effects and interactions of gonadotropin-releasing hormone (GnRH) analogues (lecirelin and buserelin), administration routes (intramuscular and epidural), and initial estrus cycle phases (follicular and luteal) on fertility outcomes in cyclic dairy cows subjected to the Ovsynch protocol. RESULTS: A total of 406 multiparous Holstein cows were randomly assigned to four groups: intramuscular lecirelin (L, n&#x2009;=&#x2009;102), intramuscular buserelin (B, n&#x2009;=&#x2009;102), epidural lecirelin (L, n&#x2009;=&#x2009;101), and epidural buserelin (B, n&#x2009;=&#x2009;101). During the Ovsynch protocol, the first GnRH treatment was administered on D0, followed by PGF&#x3b1; on D7, the second GnRH treatment on D9, and fixed-time artificial insemination (FTAI) 20&#xa0;h later on D10. At each GnRH administration, cows received either 62.5&#xa0;&#xb5;g lecirelin acetate or 10&#xa0;&#xb5;g buserelin acetate by the intramuscular or epidural route. Blood samples for serum progesterone (P) determination were collected on D0, D7, D9, D10, and D13. The initial cyclic phase was determined on D0 by ovarian ultrasonography and serum Pconcentration. Pregnancy status was assessed on D30 after FTAI, and pregnancy losses on D60. No significant differences in pregnancy or pregnancy-loss rates were detected between GnRH analogues or routes of administration, irrespective of the initial cyclic phase (P&#x2009;>&#x2009;0.05). Likewise, within each initial cyclic phase, the route of administration did not significantly affect pregnancy rate (P&#x2009;>&#x2009;0.05), although opposite numerical trends were observed. Pregnancy rates were numerically higher after intramuscular administration in cows starting the protocol during the luteal phase (44.31% vs. 31.95%, P&#x2009;=&#x2009;0.083), but numerically higher after epidural administration in those starting during the follicular phase (46.66% vs. 34.48%, P&#x2009;=&#x2009;0.083). Epidural GnRH administration was also associated with higher Pconcentrations on D13 (P&#x2009;<&#x2009;0.001). CONCLUSION: Consequently, although changing the GnRH analogue and administration route did not significantly affect overall fertility outcomes, the findings suggest a possible phase-dependent relationship between administration route and pregnancy outcome. However, this trend requires confirmation in further studies.