## 2 Materials and Methods
### Statistical Analysis  
The gestation stage when the VIT was implanted was determined by an experienced veterinarian upon rectal palpation and a gestation of 282 days was assumed to determine the calving date. The number of days that the VIT remained implanted was calculated as the date of confirmed VIT expulsion minus the date of implantation. The date of the VIT expulsion was confirmed from the data retrieved from the VIT, as well as daily visual and VHF tracking of each cows' VIT over the calving period. The number of days that the VIT was expected to remain implanted was calculated as expected calving date minus VIT implant date. In addition, the number of days between observed and expected calving date were calculated for those animals with calf sighting confirmed upon birth. Descriptive statistics for vaginal temperature was calculated, while the VIT remained implanted in the cow and included external temperature and the VIT was expelled to understand the potential impact \begin{table}
\begin{tabular}{l l l} \hline  & **Brand 1** & **Brand 2** \\ \hline \multirow{4}{*}{Communication} & Each collar can receive birth alert pulses from up to 8 VIT & Only paired VIT and collar can communicate \\  & UHF between collar and VIT & UHF between collar and VIT \\  & VHF between VIT and Yagi antennae & VHF between VIT and Yagi antennae \\  & Collars send data through Hidium Satellite system & Collars send data through Hidium Satellite system \\ \hline \multirow{2}{*}{Expelled Alert} & No activity for 60 min and \(\pm\)3 °C from benchmark & \\  & temperature, or no activity for \(>\)12 h & No activity for 50 min and VIT temperature \(<\)34 °C \\ \hline \multirow{2}{*}{Birth alert message} & Emal alert with geolocation & Emal alert with no geolocation \\  & GSM with geolocation & GSM with no geolocation \\ \hline \multirow{2}{*}{Geolocation data} & Website and on-board & Website, software and on-board 1 \\ \hline \multirow{2}{*}{Data accessible remotely} & Geolocation, temperature, date and time at a scheduled upload interval & Geolocation, temperature, date and time at a scheduled upload interval \\ \hline \multirow{4}{*}{Data stored on board of collar} & 5-min summarized accelerometer data, geolocation, date and time, DOP 1 , D3D 2  & Expected No contact \\ \hline \multirow{2}{*}{VIT data availability} & \multirow{2}{*}{
\begin{tabular}{l} No temperature or accelerometer data stored in VIT \\ expelled alert on board and in software \\ \end{tabular} } & \\  & & \\ \hline \multirow{2}{*}{Alerts generated} & \multirow{2}{*}{Expelled} & Expelled \\  & No contact \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the two birth alert systems from vaginal implant tags (VIT) used in pregnant cows in the extensive beef cattle systems of northern Australia. of ambient conditions on missed or false alerts. The time lapse to receive a birth alert was calculated by the difference between the date and time and the expelled alert was recorded on the collar and the time the email and GSM were received. This was calculated for all animals that had both a true or false alert. The distance travelled by cows between the time when the expelled alert was registered by the collar and the time the email notification was received was calculated as the distance between both geolocations. The distance travelled by cows during the 1 h period after the expelled alert was registered was also calculated. The performance of the VIT was assessed using a confusion matrix: true alert or true birth (true positive; TP), false alert (false positive; FP), missed birth (false negative; FN), and true no alert (true negative; TN). A true positive was defined as a birth alert received and the newborn calf confirmed by sighting within the first 24 h after receiving the alert. A true negative was when a birth alert was not received, and no calf was sighted on the paddock. A false positive was defined as a birth alert received but no calf being sighted and the cow showing clear evidence of still being pregnant [21,22].