B.L. Perry

Influence of Standing Estrus Before an Injection of GnRH During a Beef Cattle Fixed-time AI Protocol on LH Release, Subsequent Concentrations of Progesterone, and Steriodogenic Enzyme Expression

Beef cows that exhibit estrus before fixed-time AI have been reported to have increased pregnancy success and increased concentrations of progesterone during the subsequent estrous cycle. Therefore, these experiments were conducted to evaluate if initiation of standing estrus before an injection of GnRH during a fixed-time AI protocol affected LH pulses, subsequent concentrations of progesterone, and luteal steroidogenic enzyme expression. In Experiments 1 and 2, cows were treated with the CO-Synch protocol (100 μg GnRH day -9, 25 mg PGF(2α) day -2, and 100 μg GnRH day 0) and allotted to one of two treatments: 1) cows that initiated estrus before GnRH on day 0 (estrus; n = 5) or 2) cows that did not initiate estrus and were induced to ovulate by the GnRH on day 0 (no estrus; n = 5). In Experiment 1, blood samples were collected at 15-min intervals from 0 to 6 (bleed 1), 12 to 20 (bleed 2), 26 to 34 (bleed 3), and 40 to 48 (bleed 4) h after GnRH. Daily blood samples were collected for 17 d. Initiation of estrus before the GnRH injection had no effect on LH release or the pattern of progesterone increase; however, cows detected in estrus had overall increased (P = 0.002) concentrations of progesterone compared with cows not in estrus. In Experiment 2, estrus was detected with the HeatWatch system. Location and size of the ovulatory follicle was determined on day 0 by transrectal ultrasonography at time of injection with GnRH. Blood samples were collected on days 3, 4, 5, 7, and 9; luteal tissue was collected on day 10 (n = 4 estrus and n = 9 no estrus) from corpus luteum (CL) originating from similar-sized follicles (13.0 to 16.0 mm). Total cellular RNA was extracted, and relative mRNA levels were determined by real-time reverse transcription PCR and corrected for GAPDH. There was no effect of estrus on CL weight or concentrations of progesterone. In addition, there was no effect of estrus, follicle size, or CL weight on luteal expression of LH receptor, StAR, CYP11A1, or 3βHSD. However, there was a correlation between follicle size and CL weight (P = 0.01; R(2) = 0.43); for every increase of 1 mm in follicle size, CL weight increased by 1.5 g. In summary, estrus did not influence release of LH, CL weight, progesterone concentrations, or expression of steriodogenic enzymes. However, as follicle size increased, CL weight increased; therefore, both follicle size and CL weight were associated with progesterone concentrations.

Effects of GnRH treatment on initiation of pulses of LH, LH release, and subsequent concentrations of progesterone

Progesterone is essential for establishment and maintenance of pregnancy. One proposed method to increase progesterone is administering GnRH at insemination. However, this method has resulted in conflicting results. Therefore, 2 experiments were conducted to evaluate how administering GnRH at insemination affected pulses of luteinizing hormone (LH) and subsequent progesterone. In Experiment 1, cows were allotted to 2 treatments: (1) GnRH (100 microg) given approximately 12h after initiation of estrus (n=5); and (2) Control (n=5). Blood samples were collected at 15-min intervals for 6h at 12 (blood sampling period 1), 26 (blood sampling period 2), 40 (blood sampling period 3), 54 (blood sampling period 4), and 68 (blood sampling period 5) h after onset of estrus. Daily blood samples were collected for 17 d. In Experiment 2, cows were allotted into 2 treatments: GnRH administered 10 to 11h (n=10) or 14 to 15 h (n=10) after onset of estrus. Daily blood samples were collected for 17 d. Cows treated with GnRH tended (P<or=0.075) to have greater LH release during blood sampling period 1, tended (P=0.095) to have fewer pulses during blood sampling period 2, tended (P=0.067) to have greater concentrations of progesterone, and had an earlier (P=0.05) increase in progesterone than control cows. Cows treated with GnRH 10 to 11h after onset of estrus had greater (P=0.01) progesterone and an earlier (P=0.04) increase in progesterone than cows treated 14 to 15 h. In conclusion, timing of GnRH treatment following onset of estrus influenced pulses of LH and subsequent progesterone.