Effects of Injecting GnRH 48 Hours after PGF2α on the Dynamic Follicular and Luteal Endocrine Cells in Post - Pubertal Holstein Heifers

Eighteen cycling Holstein heifers were allotted at random by weight and body condition score to one of two treatments to evaluate the effects of GnRH on luteal response when injected 48 hours (h) after the first injection in a 10 day interval between two i njections of PGF2α . Heifers in the control group (n=9) received an injection of saline 48 h after the first injection of PGF2α; however, heifers in the μg) 48treatmenthafter group n=9) received an injection of GnRH (100 the first injection of PGF2α. Heife rs were checked for estrus 3 times daily for 60 minutes each time. Blood samples were collected for analysis of progesterone on d ays 0 (first injection of PGF2α), 2 (48 h after the fi rst injection), 10 ( at the second injection of PGF2) and at day 17 (7 days a fte r the second injection of PGF2α ). Plasma samples were analyzed fo r concentration of progesterone via radioimmunoassay to evaluate luteal cell function. Concentrations of progesterone did not differ between the control heifers and treatment animals at any day of the study. However, heifers treated with GnRH showed a significant decline in concentration of progesterone from day 0 to day 2 in a luteal response to the injection; nevertheless, concentrations of progesterone increased significantly from day 2 t o day 10. Thus, this data demonstrated that injecting GnRH 48 h after PGF2α either speeds up formation of new corpus luteum or prevents full regression of the corpus luteum present prior to the first injection of PGF2α.