Prolonged Elevated Concentrations of Estradiol Do Not Affect Conception Rates in Beef Cattle

Following treatments causing either prolonged elevated concentrations of estradiol associated with development of persistent follicles or inhibited elevated concentrations of estradiol and development of persistent follicles, conception rates were compared. Beef females received either four norgestomet implants for 9 days (day 0 = treatment initiation; n=59) or one norgestomet implant for 7 days and three additional norgestomet implants for 2 days (n=60). All implants were removed on day 9 followed by estrous detection and AI for 7 days. Treatment and day interacted to affect estradiol concentrations from day 0 to day 9 with elevated estradiol in females treated with one norgestomet implant for 7 days. Conception rates to AI were similar across treatments. Prolonged elevated concentrations of estradiol associated with development of persistent ovarian follicles do not affect fertility when persistent ovarian follicles are not allowed to ovulate

A Novel Estrous Synchronization Program for Beef Cattle Using Melengestrol Acetate

Estrous synchronization rate and conception and pregnancy rates to AI were evaluated following three estrous synchronization protocols for beef cattle. During 1995 and 1996, heifers and cows (n = 379) received either: 1) melengestrol acetate (MGA) for 18 days plus an injection of progesterone and estradiol in oil 7 days before end of treatment; 2) MGA for 17 days; or 3) two injections of PGF2&#;&#;&#;10 days apart. The greatest pregnancy rates (number conceived/number treated) among both anestrous and estrual females were achieved following treatment with MGA and an injection of progesterone and estradiol

Regulation of LH Secretion by Progesterone in Heifers

Heifers experienced either a: 1) large magnitude of change in progesterone; 2) medium magnitude of change in progesterone; or 3) small magnitude of change in progesterone. During the 24 hours following the progesterone shift, heifers with the large magnitude progesterone shift had a greater LH pulse frequency than heifers with a medium or small magnitude of shift in progesterone. Despite the large or medium magnitude progesterone shift, LH pulse frequency did not differ from heifers in which a small change in progesterone occurred. We conclude that amount of progesterone in circulation is more important in regulation of LH secretion than magnitude of shift in amount of progesterone