The use of birthing sensors in controlled dairy environments have been effectively employed to increase cow and calf survival at birth through prompt intervention of dystocia events. Yet, they have not been developed or utilized in large-scale environments (i.e., ranching operations) where cattle are managed using free-grazing production systems. This study evaluated vaginal birthing sensors which were specifically developed as a calving alert system for cows in large-scale environments. We deployed a total of 297 birthing sensors in dams on three Florida ranches from August 2017 to May 2018 in pastures from 0.36 to 0.55 km² in area. Average length of sensor deployments was 72.1 days (range 0.5–146 days) with a total of 140 (47.1%) sensors failing to send an alert upon expulsion. Of the 140 failed sensors, 77 were tested for sensor power and 50 were out of power. The likelihood of a successful alert was negatively associated with the time sensors were deployed. At best, the modelled probability of success was 81.8% at day zero and reduced to 63.8% at day 50. Failures were identified to likely be caused by a variety of reasons, including issues with software protocols, battery life expectancy, and possible malfunction of hardware components. Based on these results, the birthing sensor technology utilized in this study can work in the expansive pasture system, but currently is not an economical or logistically reliable tool to monitor and manage calving due to failure rates. We do recommend that development of intravaginal birthing sensors for these conditions is possible, with a focus on extending battery life to support increased signal transmission for longer range communication at the extreme of equipment capabilities, as well as improving software communication protocols among technological components.