The objective of the study was to validate the ear tag (Smartbow GmbH, Weibern, Austria) for grazing behavior. The Smartbow ear tag includes an acceleration sensor, a radio chip, and temperature sensor for calibration. Smartbow can monitor estrus detection and rumination by acceleration data from ear and head movements. To validate Smartbow, a halter system (Rumiwatch, Itin and Hoch GmbH, Liestal, Switzerland) was used. The halter is comprised of a 3-axis accelerometer that records acceleration patterns by a noseband pressure sensor and detects jaw movements according to chewing activities. The study was conducted at the University of Minnesota grazing dairy in Morris, Minnesota, and the Teagasc Animal and Grassland Research Centre in Ireland. During May and June of 2017, Smartbow ear tags were attached to grazing cows and 3 observers visually recorded behaviors for a total of 90 h in Minnesota. Observational data from Minnesota and additional data from Ireland were used to develop a grazing algorithm. During September of 2018, data were collected by Smartbow and Rumiwatch with 12 crossbred cows in Minnesota (n = 248 h) and 10 Holstein Friesian cows in Ireland (n = 248 h). A 2-sided paired t-test compared the percentage of time recorded for grazing behaviors and Pearson correlations (PROC CORR of SAS) evaluated associations between Smartbow and Rumiwatch. For total recorded grazing time in Minnesota, the percentage of time recorded by Smartbow was 37.0% (CI 32.1 to 42.0) and by Rumiwatch was 40.5% (CI 35.5 to 45.6). For total recorded grazing time in Ireland, the percentage of time recorded by Smartbow was 35.4% (CI 30.6 to 40.2) and by Rumiwatch was 36.9% (CI 32.1 to 41.8). Smartbow and Rumiwatch agreed strongly for monitoring grazing in Minnesota (r = 0.96; CI 0.94–0.97; P < 0.01) and in Ireland (r = 0.92; CI 0.90–0.94; P < 0.01). The results suggest that there is great potential for Smartbow to be utilized in pasture-based dairy production systems to support farm management decision making.