In Experiment 2, the effect of the occurrence of estrus on ovarian responses was evaluated in cows (N 53) synchronized using ECP at device removal. Cows that displayed estrus had a greater diameter of the largest follicle (LF) at device removal (P 0.0001), a greater diameter at TAI (P
0.0001), a greater ovulation rate (P 0.02), a larger CL (P 0.02), and a greater P4 concentration (P 0.0001) than cows that did
not display estrus. In Experiment 3, the effect of GnRH treatment on P/AI at TAI was evaluated in cows that received ECP at device
removal, and either displayed, or did not display, estrus (N 726). There was no estrus by GnRH interaction (P 0.22); the P/AI was
greater (P 0.0001) in cows that displayed estrus (61.9%) than cows that did not display estrus (41.4%). However, GnRH did not
improve (P0.81) P/AI (GnRH53.7% vs. no GnRH52.6%). In conclusion, exogenous estradiol at device removal increased both
the proportion of suckled Bos indicus cows that displayed estrus and P/AI. Cows that displayed estrus had better ovarian responses (i.e.,
larger follicles at TAI, a greater ovulation rate, larger CL, and greater P4 concentrations) following an estradiol/P4-based synchronization
protocol. Although occurrence of estrus improved pregnancy outcomes, GnRH at TAI did not improve P/AI in suckled Bos indicus cows
treated with ECP, regardless of estrus occurrence.