Effects of human chorionic gonadotropin (hCG) pretreatment on endocrine and behavioural responses of dairy cattle to exogenous prostaglandin F2α

Abstract Sixteen heifers were used in two experiments to examine the effects of exogenous luteotropic stimulation on endocrine and behavioural responses to prostaglandin F 2α (PGF 2α ). In each experiment, eight heifers were randomly assigned at synchronized oestrus to either receive 2500 IU of human chorionic gonadotropin (hCG) 48 h later or be maintained as controls. Blood samples for plasma progesterone (P) determinations were taken every 12 h from oestrus until PGF 2α administration on Day 7 (Experiment 1) or Day 13 (Experiment 2). After PGF 2α , samples were collected every 6 h for the next 36 h, and then every 2 h from 38 to 72 h. These samples were analyzed for plasma (P), luteinizing hormone (LH), and oestradiol 17 β (E 2 ) by radioimmunoassay. Administration of hCG 48 h postoestrus did not affect plasma P levels prior to PGF 2α administration in either experiment. In contrast, plasma E 2 levels were higher in control than hCG-treated heifers from 12 to 50 h post-PGF 2α in Experiment 1 and 12–68 h in Experiment 2. All eight control heifers exhibited a preovulatory LH surge during the first 72 h following PGF 2α administration, while one of the four hCG-treated heifers in each experiment had an increase in plasma LH levels. Of the hCG-treated group, one of four heifers in Experiment 1 and two of four heifers in Experiment 2 were observed in oestrus during the 96 h following PGF 2α , while all control heifers exhibited oestrual behaviour during this period. Exogenous administration of hCG during the early luteal phase did not affect subsequent luteolytic responses of the corpus luteum to PGF 2α , but did significantly reduce plasma E 2 levels, LH secretion, and delayed behavioural oestrus responses following PGF 2α .