Tail chalk improved detection of true estrous alerts from an automated activity monitor system in a cohort study

This study aimed to assess how two estrus detection tools, an automated activity monitor (AAM) and tail chalk (TC), were associated with true estrus events in lactating Holstein cows. Cows with days in milk ≥ 5 were monitored twice daily for estrus alerts, identified by the AAM and a TC score of 3 (< 50% of chalk remaining). Transrectal ultrasonography was performed on the day of alert to confirm estrus (cows with a follicle ≥10 mm and corpus luteum ≤24 mm). For AAM‐identified events, peak activity and duration of estrus were measured. Mixed logistic and linear regression models were used to assess associations for dichotomous and continuous outcomes, respectively. A total of 1,714 events were recorded, of which 1,098 from 371 cows were considered in true estrus (AAM only, n = 629; TC only, n = 86; AAM+TC, n = 383). Cows with both AAM and TC alerts had higher odds of true estrus compared to those with only AAM alerts (OR = 4.24; 95%CI = 3.05–5.87) or only TC alerts (OR = 17.70; 95%CI = 11.90–26.34). Cows having AAM and TC alert had greater peak activity index (AAM only = 221.64 ± 7.07; AAM+TC = 323.89 ± 9.13) and longer duration of estrus (AAM only = 8.37 ± 0.27h; AAM+TC = 13.17 ± 0.33h) compared to cows with AAM alerts only. In conclusion, TC could be used as a marker of high-intensity estrus and, when combined with an AAM, improves identification of true estrus in dairy cows.