Potential Modification of the PG 6-day CIDR Protocol for Herds with 50% Bos Indicus-influence

Abstract Estrous synchronization protocols have been utilized to increase conception rates in beef cattle. There is currently limited research on protocols for 50% Bos indicus influence cattle. This study was conducted to evaluate whether administration of a double dose of GnRH (200 μg) at either CIDR insertion or 3 days prior would enhance estrus expression and conception rates in Bos indicus (50% to 100% Brahman) cattle compared to Bos taurus (Simmental) cattle. Data were collected from three breeding seasons (Fall and Spring 2024, Spring 2025). Simmentals (n = 56) were synchronized using the PG 6-day CIDR protocol (PGF2a on d -9; GnRH and CIDR insertion on d -6; and PGF2a and CIDR removal on d 0). Bos indicus cattle received either GnRH (200 µg) and PGF2a (PGF+2xGnRH; n = 109), or PGF2a only (PGF; n = 135) on d -9. At CIDR insertion (d -6), PGF cattle received GnRH (200 µg), whereas the PGF+2xGnRH group received no injections. On d 0, CIDRs were removed, and all animals received an injection of PGF2a. Estrus detection was performed on d 1 to d 3 using EstroTect patches and visual assessment. Animals were inseminated by the am/pm rule, and animals not detected in estrus were cleanup AIed on d 4 with an injection of GnRH concurrent with AI (100 µg – Simmental, 200 µg - Bos indicus). Differences in estrus and pregnancy success were analyzed using the GLIMMIX procedure in SAS (v9.4). Age, days postpartum, cycle status, estrus, semen type (sexed vs. conventional), and body condition score (BCS) were accounted for in the model. Simmentals exhibited significantly greater estrus expression compared to Bos indicus (P 0.01; 78% vs 47%), but cycling status (P = 0.02) and body condition (P = 0.03) also influenced estrus expression. Animals that were cycling exhibited greater (P = 0.02) estrus than anestrous animals (69% vs 49%), and estrus expression was decreased in condition score 6 animals vs 5 or 5.5 (43% vs 67% or 68%). There was no difference (P = 0.19) between PGF+2xGnRH and PGF (43 vs 52%). Pregnancy success was influenced by estrus expression (P 0.01) and semen type (P = 0.02), and there was a tendency for a semen type by estrus expression interaction (P = 0.09), but treatment did not influence pregnancy success (P = 0.72). Overall, a double dose of GnRH did not significantly enhance estrus expression in Bos indicus influenced cattle when compared to Bos taurus, but overall pregnancy success did not differ between treatments.