Effects of the formation and regression of accessory corpus lutea during pregnancy on plasma progesterone concentration and pregnancy status in cross-bred beef heifers.

Abstract There was examination of effects of accessory corpus lutea (CLs) formation and regression during pregnancy on circulating progesterone (P4) concentrations and pregnancy maintenance in beef heifers. Heifers (Experiment 1, n = 75; Experiment 2, n = 496) were randomly assigned to either a human chorionic gonadotropin (hCG) treatment or untreated group 5 days post-estrus, followed by embryo transfer (ET) on Days 6–8 (Day 0 = Estrus). In Experiment 1, blood samples were collected from pregnant heifers on Days 33, 40, and 47 for conducting P4 assays. Plasma P4 concentrations were greater in hCG-treated heifers than in untreated heifers on Day 33. In hCG-treated heifers with accessory CL regression between Days 33 and 47, plasma P4 decreased to concentrations similar to those of untreated heifers after Day 40. In hCG-treated pregnant heifers in Experiment 2, CL regression by Day 50 of gestation was more frequent when CLs were contralateral (49.3 %) rather than ipsilateral (4.4 %, P