The influence of estrus and double ovulations of Holstein heifers on a 5d-Co-synch 72 h protocol during summer was explored. Cycling heifers on the 5d-Co-72 h synchronisation protocol were randomised to carry a progesterone device (116) from day 0 to day 5 of the synchronisation protocol or not (97), from them 100 and 84 underwent fixed-time artificial insemination (FTAI). Another 70 heifers showed estrus before the first GnRH and were inseminated. Heifers were pregnant 34% (33/97) without device, 44.0% (51/116) with device and 44.3% (31/70) inseminated after estrus. Pregnancy per insemination (P/AI) tended to be higher (p = 0.059) inseminating after estrus than after FTAI without device. Three heifers without device and two with it showed double ovulation, four of which became pregnant. Pregnancy loss was similar among groups (0%-10%). P/AI was higher in heifers with estrus before FTAI (OR 13.96, p = 0.027), and with each additional day of age (OR 1.015
p = 0.015). The device was associated with higher progesterone on day 5, with more synchronised heifers, more ovulation frequency after first GnRH (96.6% vs. 84%, p = 0.007), and more heifers showing estrus before FTAI (96.0% vs. 85.7%, p = 0.018). Days to conception after first AI of the study, and after maximum three AIs were similar between synchronised groups, but significantly shorter in heifers inseminated after observed estrus. Fertility was higher in heifers that showed signs of estrus before accompanying the FTAI, while double ovulation seems not to be a major factor for fertility. An intravaginal device slightly improves efficiency under heat stress by increasing ovulation, progesterone, proportion of synchronised heifers and heifers showing estrus at AI.