Natural versus induced estrus indicators of Holstein heifers determined by an automated activity monitoring system

The objective of this study was to determine the association between estrus induction and estrus indicators, as well as the association between estrus indicators and subsequent fertility. A total of 609 Holstein heifers were fitted with an ear tag automated activity monitor (AAM; SCR by Allflex) and each estrus event was categorized as natural or induced. An induced estrus event was defined as an estrus occurring within 2 to 5 d of PRID removal and/or prostaglandin administration. Most heifers received sex-sorted semen for the first AI and conventional semen for subsequent AIs, with 6 opportunities to become pregnant. Pregnancy was diagnosed by ultrasound at 30 d and confirmed at 45 d post-AI. The AAM recorded the date and time of onset, peak and end of high activity, heat index (a measure of estrus strength), maximum activity change, maximum rumination change, and duration of high activity. Data from 1,269 estrus events were included in the analyses, 881 from natural and 388 from induced estrus events. In total, 70% of heifers started high activity between 20:00 and 08:00 and 73% of heifers hit peak activity between 02:00 to 14:00. Heat index (82 vs. 80, p = 0.09) and rumination change (-49 vs. -46 min/d, p = 0.06) tended to be greater for induced vs. natural estrus. There was no difference in pregnancy at 30 d (68 vs. 67%, p = 0.79) or 45 d (64 vs. 63%, p = 0.82) between the induced and natural estrus groups. However, for pregnancy at 30 d post-AI using conventional semen, natural estrus events tended to have greater pregnancy per AI (P/AI) than induced estrus events (72 vs. 63%; p = 0.09). For pregnancy at 45 d post-AI, induced estrus with sex-sorted semen tended to have greater pregnancy per AI compared to induced estrus with conventional semen (68 vs. 58%; p = 0.09). There was no difference in pregnancy loss between estrus types (p = 0.38). When comparing estrus events that resulted in pregnant or non-pregnant diagnoses at 30 d post-AI, the only difference in estrus indicators was for estrus events resulting in pregnancy to have a greater rumination change (-50 vs. -46 min/d; p = 0.04). Overall, the induction of estrus had no negative effects on estrus indicators, as measured by an AAM, or on fertility compared with natural estrus in dairy heifers.