Luteinizing hormone and progesterone concentrations in serum of heifers administered a short half-life prostaglandin (PGF_2α) or long half-life prostaglandin analogue (fenprostalene) on days six or eleven of the estrous cycle

Two trials were conducted to measure the progesterone (P₄) decline and luteinizing hormone (LH) surge in serum subsequent to administration of a short half-life (short t _{1 2}) prostaglandin (PGF_2α) or a long half-life (long t _{1 2}) prostaglandin analogue (fenprostalene) on Days 6 or 11 of the estrous cycle. Twenty-five crossbred Shorthorn and five Hereford heifers with a mean weight of 331.4 ± 29.8 kg were used in both trials. The heifers were randomly allotted to receive either a short t _{1 2} or long t _{1 2} prostaglandin treatment on Day 6 or 11 of the estrous cycle. A crossover design for the main effect, treatment (type of prostaglandin), was conducted. Heifers that received PGF_2α in Trial I were given fenprostalene in Trial II and vice versa. Stage of the estrous cycle (day) was the same for each heifer in both trials. Stage of estrous cycle was standardized to either Day 6 or 11 by administering Syncro-Mate B (SMB). Blood was collected every hour for 80 h post injection to quantify LH and P₄ concentrations. There were no significant differences (P > 0.05) between the short t _{1 2} or long t _{1 2} for either P₄ or LH profiles. In addition, no differences were detected between stages of the estrous cycle for the timing of the preovulatory surge of LH after prostaglandin administration.