## 4 Discussion
Therefore, these results suggest that deployments over three months are not feasible at present and VIT should be inserted between one and three months prior to calving to improve reliability. The VIT retention issues in deer were also evident in the work by [PERSON] et al. [35], who suggested that the wing length of the VIT and the geographic location of the animals may have an impact on retention rates. The effects of long term intravaginal device placement has not been widely studied in cattle, however a study by [PERSON] et al. [36] showed no physical or physiological impacts after 107 days implantation of vaginal temperature transmitters in 18 Holstein heifers. The design of the temperature transmitters used by [PERSON] et al. were composed of multiple anchoring 'fingers' stabilizing the transmitter inside the vagina. The design (fingers or wings) and length of the VIT implantation may result in vaginal contractions, which may have caused early VIT expulsion. However, in a study that focused on the welfare of clk fitted with VIT [15], there was no evidence that the VIT had a negative impact on reproductive performance. Furthermore, there were no signs of birthing complications in animals fitted with VIT, which was also the case in this trial. In Trial 2, the VIT devices were meant to be implanted for four months, but on average these were expelled at approximately three months. Unlike other studies using vaginal temperature sensors for cattle to detect calving [16], this study required an extended period of implantation before birth, which may have impacted its effectiveness. In addition to the early expulsion seen in Trial 2, the VIT used up their on-board data storage capacity within three months after insertion, and therefore, temperature and activity data were not recorded after this period. However, the VIT and collar were still able to communicate, and birth alerts were possible. Therefore, the design, size, and battery life of VIT to operate and be retained for extended periods of time in cattle need further research and improvement for applications. The main limitation of the birth alert systems was the high number of missed and false alerts, which could have been caused by a range of environmental and technological factors. Missed alerts could occur due to failed UHF communication between the collar and the VIT. For example, the alert may be missed if a cow moved out of UHF range of the expelled VIT before the alert was triggered at 50 or 60 min depending on the brand. The fact that cows travelled up to 13 km on the day of calving and up to 643 m during the first hour after the expelled alert in Trial 1 confirms this could have been a major contributing factor to missed birth alerts. A missed alert could also occur if the VIT did not meet the required temperature and activity thresholds. With Brand 1 VIT, a change in temperature less than \(\pm 3~{}^{\circ}\)C from the benchmark established for the first week after insertion did not trigger a birth alert and it assumed that the VIT was still inside the cow. The missed alerts for Brand 1 must be interpreted with caution because three of the four missed alerts were due to a calving event occurring inside this one-week calibration period. However, no animals in the trial were fetal aged more than 8.5 months pregnant at the time of VIT insertion, and thus, Brand 1 missed alerts were due to the cows calving earlier than expected rather than a fault in the VIT. The reliance on temperature changes in the VIT to trigger alerts could be a limitation of this technology [23] where the ambient temperature may be close to the animal's body temperature, such as in tropical or subtropical conditions. If animals were placed in housing conditions where the temperature was lower and there were shaded areas, the likelihood of the birth alert success would likely be higher [16; 31].