Comparison of luteolysis and timed artificial insemination pregnancy rates after administration of PGF_2α in the muscle or the ischiorectal fossa in cattle

Prostaglandin F_2α (PGF_2α) is commonly injected intramuscularly (IM) in female cattle in synchronization protocols. A novel site for administration of PGF_2α that improves beef quality assurance is the ischiorectal fossa (IRF). The objective of this study was to determine whether administration of PGF_2α in the IRF results in a similar physiological response to an intramuscular injection. Yearling angus-cross heifers (n = 112) were blocked by weight and randomly assigned within blocks to be injected with 5 mL PGF_2α either IM in the neck or in the IRF. Blood samples taken at 0, 8, 16, 24, 36, and 48 h post-injection were analyzed for serum progesterone concentration using a radioimmunoassay. Progesterone concentration curves for each heifer were plotted to determine luteolysis. The median times to luteolysis for neck and IRF injections were 18.1 h and 20.0 h, respectively (p = 0.06). Angus cross commercial beef cows (n = 1471) at least 30 days post-partum were blocked by age and randomly assigned within blocks to be injected with 5 mL PGF_2α either IM in the neck muscle or in IRF as part of a 7-Day CO-Synch + CIDR synchronization protocol. Pregnancy diagnosis was performed via ultrasound at 60 days post insemination. Results were analyzed with Proc Glimmix (SAS). Pregnancy rates for neck and IRF injections were 52.6% and 57.2%, respectively (p = 0.06). In summary, injection of PGF_2α in the IRF for synchronization of estrus and luteolysis did not differ from IM injection. Utilizing the ischiorectal fossa as an injection site for PGF_2α may serve as an alternative that more closely aligns with beef quality assurance.