Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Regulation of LH secretion by progestins and their use in estrous synchronization programs in beef cattle
Abstract
The first experiment evaluated the role of magnitude of change in concentration of progesterone in regulation of LH secretion. Heifers were treated with various doses of progesterone followed by treatment with a large dose of progesterone. Based on magnitude of change in concentration of progesterone, heifers were placed in one of three groups; (1) large magnitude of change in progesterone, (2) medium magnitude of change in progesterone, or (3) small change in progesterone. Despite a large or medium magnitude of change in progesterone, LH pulse frequency was similar to heifers in which a small magnitude of change in progesterone occurred. We conclude that absolute concentration of progesterone is more important in regulation of LH secretion than magnitude of change in concentration of progesterone. The objective of the second experiment was to determine if elevated concentrations of 17$\beta$-estradiol associated with development of persistent ovarian follicles affect fertility if persistent ovarian follicles are not allowed to ovulate. Beef females received either 4 norgestomet implants from d 0 to 9 (d 0 = treatment initiation) or 1 norgestomet implant from d 0 to 7 and 3 additional implants from d 7 to 9. Estradiol increased from d 0 to 7 in heifers that received 1 plus 3 norgestomet implants, then decreased on d 7 indicating the development and induced regression of persistent ovarian follicles. Conception rates to AI were similar across treatments. We conclude that elevated concentrations of 17$\beta$-estradiol do not affect fertility when persistent ovarian follicles are not allowed to ovulate. The third experiment compared effectiveness of three estrous synchronization protocols for beef cattle. Estrous cycles were synchronized by either; (1) feeding MGA (0.5 mg$\cdot$animal day$\sp{-1}$) for 18 d, (2) feeding MGA for 18 d plus 200 mg progesterone and 1 mg 17$\beta$-estradiol (E$\sb2$) injected 7 d before cessation of MGA (MGA + P$\sb4$), or (3) two injections of PGF$\sb{2\alpha}$ 10 d apart (PG). Among anestrous females, estrous synchrony rates were greatest among females treated with MGA + P$\sb4$. Among estrual females, estrous synchrony rates were greater among females treated with MGA + P$\sb4$ or PG as compared with MGA alone. Conception rates were greater following treatment with either PGF$\sb{2\alpha}$ or MGA + P$\sb4$ as compared with MGA alone. Among anestrous or estrual females treated with MGA + P$\sb4$, pregnancy rates were greater than or similar to those treated with PGF$\sb{2\alpha}$ or MGA alone. Estrous synchronization of beef cattle using MGA + P$\sb4$ and E$\sb2$ differentially improves estrous synchronization and pregnancy rates while maintaining typical conception rates.
Subject Area
Anatomy & physiology|Animals|Livestock
Recommended Citation
Fike, Karol Elizabeth, "Regulation of LH secretion by progestins and their use in estrous synchronization programs in beef cattle" (1997). ETD collection for University of Nebraska-Lincoln. AAI9734616.
https://digitalcommons.unl.edu/dissertations/AAI9734616