Document Type

Dissertation - Open Access

Award Date

2020

Degree Name

Doctor of Philosophy (PhD)

Department

Animal Science

First Advisor

George A. Perry

Keywords

Beef Cattle, Estradiol, GnRH, Granulosa Cell, Ovary

Abstract

Previous studies evaluating single and multiple GnRH injections at time of CIDR removal/prostaglandin injection using the 7-day CO-Synch + CIDR protocol have reported decreased interval to estrus and increased expression of estrus, or increased conception rates, respectively. Thus, the objective of experiment 1 (Chapter 3) was to compare the effects of supplementing GnRH at prostaglandin injection on expression of estrus and conception rates in beef cows and heifers synchronized with different protocols. The objective was conducted in two parts, in study 1, Beef cows (n = 161) and heifers (n = 1803) from 16 herds over three years were synchronized using the 7-day COSynch + CIDR fixed time AI (FTAI) protocol or the MGA-PG FTAI heifer protocol and were randomly assigned to receive one of three treatments at prostaglandin injection [0 μg GnRH (0); 5 μg GnRH (5); or 5 μg at prostaglandin injection and 5 μg 12 h later (5+5)]. In study 2, Beef cows (n = 656) and heifers (n = 4552) from 24 herds over three years were synchronized using the 7-day CO-Synch + CIDR fixed time AI (FTAI) protocol or the MGA-PG FTAI heifer protocol and were randomly assigned to receive one of two treatments at prostaglandin injection [0 μg GnRH (0) or 5 μg GnRH (5)]. In Study 1, cows treated with 5 μg of GnRH tended to have increased FTAI conception rates compared to heifers. Bovine follicles with greater follicular fluid concentrations of estradiol had decreased expression of GnRH-I and GnRH-II in granulosa cells (GC). The objective of experiment 2 (Chapter 4) was to characterize relative abundance of GnRH-I, GnRH-II, and GnRH-IR mRNA within GC of follicles at specific stages of development. Across all follicles, GnRH-I and GnRH-II were not influenced by stage (P = 0.49) but were influenced by size (P < 0.0001). Smalls (4.55 ± 0.39 and 3.91 ± 0.44, respectively) had greater expression (P ≤ 0.01) compared to mediums (0.83 ± 0.39 and 1.41 ± 0.44, respectively) and larges (0.52 ± 0.47 and 2.12 ± 0.54, respectively). There was also a stage by size interaction for GnRH-I, GnRH-II, and GnRH-IR (P ≤ 0.03). POST and POST-PG smalls had increased (P ≤ 0.03) expression of GnRH-II and GnRH-IR mRNA compared to PRE smalls. POST smalls had (P = 0.02) increased expression of GnRH-I mRNA compared to PRE smalls. PRE mediums had increased GnRH-I and GnRH-II mRNA expression (P ≤ 0.03) compared to POST-PG mediums. When only the largest follicle for each animal was evaluated, stage of development influenced expression of GnRH-I (P = 0.03) but not GnRH-II (P = 0.91) or GnRH-IR (P = 0.16). For GnRH-I, PRE tended (P = 0.09; 2.28 ± 0.55) to have increased expression compared to POST (0.92 ± 0.55) and did have greater expression compared to POST-PG (P = 0.01; 0.11 ± 0.55), and POST-PG follicles had increased follicular fluid estradiol-17β concentrations (P < 0.0001) compared to PRE and POST follicles. There is limited information on the functional role of GnRH at the level of the ovary or any factors involved in its regulation. However, a considerable amount of information exists regarding hypothalamic control of GnRH. Some factors at the level of the hypothalamus that have been reported to be in opposition with or regulate GnRH and GnRH neurons are POMC and its derivatives (i.e. α-MSH), NPY, AMH, and GnRH itself. The objective of experiment 3 (Chapter 5) was to identify and characterize the localization of GnRH and factors that are known to regulate GnRH production at the level of the hypothalamus within the bovine ovary. Immunohistochemistry against α- MSH, NPY, GnRH, and AMH was performed on bovine ovarian cryosections. Colocalization of immunofluorescence for GnRH and these factors was present in the granulosa cell layer(s) of primary, secondary, and small antral follicles as well as in vessels and arteries within the stroma. Furthermore, NPY immunofluorescence was detected in nerve fibers of the ovarian stroma surrounding follicles. In summary, expression of estrus increased pregnancy success (P < 0.0001), but supplementation with 5 or 5+5 μg of GnRH at prostaglandin injection did not improve estrus response or conception rates in beef cows and heifers regardless of protocol. Granulosa cell GnRH-I and GnRH-II mRNA abundance was increased in small, early follicular stage follicles, with decreased concentrations of estradiol. The colocalization of GnRH, α-MSH, NPY, and AMH within the granulosa cell layer(s) of follicles, and within the nerve fibers and arteries within the ovarian stroma are suggestive of a local role in regulating follicular growth and ovaria steroidogenesis.

Format

application/pdf

Number of Pages

226

Publisher

South Dakota State University

Rights

In Copyright - Educational Use Permitted
http://rightsstatements.org/vocab/InC-EDU/1.0/

Available for download on Thursday, August 18, 2022

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