Effect of estradiol preceding and progesterone subsequent to ovulation on proportion of postpartum beef cows pregnant.

Two experiments were conducted to examine the effect of plasma concentrations of 17β-estradiol (E) preceding and progesterone (P) subsequent to ovulation on proportions of beef cows pregnant following embryo transfer. Timing of ovulation (d 0) among postpartum cows was synchronized and cows that expressed estrus were removed from each study. In Experiment 1, plasma E concentration on d 0 was used to classify cows (n = 353) into Low, Medium, and High E groups. Pregnancy rate for cows with Low, Medium, or High E concentrations were different (P < 0.05). In Experiment 2, there were multiple administrations of PGF to evaluate the independent effects of Low or High E before ovulation and Low or Normal (no treatment) P after ovulation on proportions of cows pregnant. Treatment groups in Experiment 2, therefore, were: Low E-Low P (LL; n = 71), Low E-Normal P (LN; n = 69), High E-Low P (HL; n = 74), and High E-Normal P (HN; n = 73). Concentrations of P on d 7 subsequent to ovulation were less (P < 0.05) in cows of the HL compared to HN, and in LL compared to LN groups. Concentrations of E on d -2, 0, and change in E (d -2 to d 0) had a positive effect (P < 0.008) on pregnancy rates. In summary, relatively greater E concentrations preceding ovulation; and relatively greater P concentrations subsequent to ovulation combined with lesser E concentrations preceding ovulation had a positive effect on proportions of postpartum cows pregnant.