Date of Graduation

1999

Document Type

Dissertation/Thesis

Abstract

The relationship of luteal function and embryonic survival to concentrations of estradiol-17β or prostaglandin (PG) F2α was evaluated after induction of replacement corpora lutea (CL) in lutectomized beef cows maintained on flurogestone acetate (FGA). On day 26 of pregnancy the CL was enucleated, and the ovary contralateral to the gravid uterine horn was removed. Pregnancy was maintained initially by the administration of FGA (6 mg, s.c.) every 12 h, beginning 12 h before lutectomy. Between days 28 and 31, when a follicle had grown to 10 mm in diameter, cows were treated with hCG (1000 IU, i.m.) to induce formation of a replacement CL. Treatment with FGA continued for 10 days after treatment with hCG. If a CL was present between the fifth and tenth days, FGA was reduced by 2 mg per day. Cows that did not form a CL initially were retreated with hCG when another follicle was ≥10 mm, during continued daily treatment with FGA. Blood samples collected every hour from 0900 to 1900 h on days 31 to 35 from catheters in the posterior vena cava were assayed for concentrations of PGF2α and luteinizing hormone (LH). Blood samples collected every morning from day 28 to 60 via jugular venipuncture were assayed for concentrations of progesterone and estradiol-17β. Viability of the embryo was determined by daily ultrasonographic observation for a heartbeat. Maintenance of pregnancy was greater in cows that formed a CL after (9/9) than before day 35 (8/16; P < 0.05). Proportion of cows forming CL to first treatment with hCG was not affected by day of treatment, by diameter of the follicle, or by concentration of estradiol-17β at treatment. Pregnancy was lost in eight cows on full replacement with FGA and in five cows after withdrawal of FGA. Maintenance of pregnancy while FGA was provided was not associated significantly with estradiol-17β (high 6/16, low 11/16) or PGF2α (high 10/13, low 5/13). Maintenance of pregnancy after FGA withdrawal in cows that formed CL in response to first hCG tended to be associated with low estradiol-17β (high 2/6, low 6/7; P = 0.10) and high PGF2α (high 5/5, low 2/6; P < 0.10) between days 31 and 35. Secretion of progesterone by replacement CL was less (P < 0.05), concentrations of estradiol-17β did not differ, and the frequency of pulses of LH was greater (P < 0.05) in cows with low compared to high PGF2α between days 31 and 35. In conclusion, maintenance of pregnancy after induction of replacement CL was increased when a CL was induced after day 35 of pregnancy. Concentrations of estradiol-17β and PGF2α during the first 5 days after treatment with hCG affected the ability to form a functional CL and to maintain pregnancy when hCG was given before day 35 of pregnancy. An experiment designed to evaluate follicular growth, synchronized estrus and pregnancy rate after fixed-time insemination is discussed in Appendix 1. An evaluation of the effect of the previously gravid uterine horn on follicular growth and pregnancy rate in early postpartum beef cows is presented in Appendix 2.

Download

Share

COinS