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》.pdfVIP

Available online at Domestic Animal Endocrinology 42 (2012) 11–19 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 S.D. Fields, K.L. Gebhart, B.L. Perry, M.G. Gonda, C.L. Wright, R.C. Bott, G.A. Perry* Department of Animal and Range Sciences, South Dakota State University, Brookings, SD 57007 Received 28 March 2011; received in revised form 15 August 2011; accepted 18 August 2011 Abstract 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 g GnRH day 9, 25 mg PGF2 day 2, and 100 g 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) concen