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
Introduction
An injection of GnRH has been reported to cause ovulation of a large dominant follicle; [1], [2] therefore, it is used at time of insemination in fixed-time artificial insemination (FTAI) protocols to synchronize ovulation [3]. However, some animals exhibit standing estrus, whereas others do not initiate standing estrus before time of FTAI, and animals that initiated estrus within 24 h of FTAI had increased pregnancy rates compared with those that did not [4], [5].
Progesterone is essential for maintenance of pregnancy and embryo survival [6]. One proposed method to increase pregnancy success through increased corpus luteum (CL) function was with an injection of GnRH at time of insemination to increase or extend the LH surge [7], [8]. However, when GnRH was given at time of insemination, there have been conflicting results in the effect on subsequent concentrations of progesterone. Studies have reported an increase [7], decrease [9], [10], or no change [11] in subsequent concentrations of progesterone compared with controls.
Luteinizing hormone has been reported to play an important role in CL development and function [8], [12], [13], and duration of the LH surge may play a role in luteal development, causing a more functional CL to be produced. One of the reasons for variation in the response to GnRH at insemination could be that pulses of LH around the time of ovulation have been reported to be important for early luteal function [12], [13], [14], and the interval from the onset of estrus to GnRH administration affected LH release around the time of ovulation [14]. Therefore, the objectives of these experiments were to determine whether initiation of standing estrus in beef cows before an injection of GnRH during a FTAI protocol affected LH pulses, subsequent concentrations of progesterone, and luteal steroidogenic enzyme expression.
Section snippets
Materials and methods
Two experiments were conducted at the South Dakota State University with the use of Angus-cross nonpregnant, nonlactating, cyclic mature beef cows. All animals were handled according to procedures approved by the South Dakota State University Institutional Animal Care and Use Committee.
Follicle size and concentrations of estradiol
Animals detected in standing estrus had larger ovulatory follicles (15.4 ± 0.76 mm; P = 0.02) than animals not detected in standing estrus (12.1 ± 0.76 mm). No effects of time, treatment, or treatment × time for concentrations of estradiol were observed during the first sampling period of intensive bleeds. Animals detected in standing estrus had 5.34 ± 0.21 pg mL−1 of estradiol, and animals not detected in standing estrus had 5.02 ± 0.21 pg mL−1 of estradiol. This indicates that cows that
Discussion
An injection of GnRH has been reported to cause ovulation of a large dominant follicle; [1], [2] therefore, it is used at time of insemination in FTAI protocols to synchronize ovulation [3]. Synchronized ovulation eliminates the need for animals to exhibit estrus or estrus detection. However, some animals exhibit standing estrus, whereas others do not initiate standing estrus, and animals that initiated estrus within 24 h of FTAI had increased pregnancy rates compared with those that did not [4]
Acknowledgments
This research was supported by the South Dakota Research Support Fund and the South Dakota Agricultural Experiment Station. The authors acknowledge the South Dakota State University Fund to Enhance Scholarly Excellence in Undergraduate Research for financial support, and the use of the SDSU-FGCF supported in part by NSF/EPSCoR grant Number 0091948 and by the State of South Dakota. Mention of a proprietary product does not constitute a guarantee or warranty of the product by South Dakota Agric.
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