In cows given estradiol (ECP or ECP GnRH), more displayed
estrus (P 0.002) and became pregnant (P 0.0001) compared with those receiving only GnRH. 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.