## 2 Materials and Methods  
All methods for this project have been approved by The University of Sydney Animal Ethics Committee (Animal Ethics Project Number 2017/1296). The study was conducted on a large commercial beef enterprise in the western Victoria River Downs Region of the Northern Territory, Australia. The area has a tropical climate, with distinct wet and dry seasons. There is an average of 800 mm of rainfall per year, predominantly falling in the wet season between December and March. The average daily maximum temperature throughout the year was 35.0 \({}^{\circ}\)C with a yearly average minimum of 21.2 \({}^{\circ}\)C [18]. There were two trials run over two calvingseasons, the first trial from April to September 2018 and the second trial from April to September 2019. Trial 1 was undertaken in a 33 km\({}^{2}\) paddock, which contained three water points. Vegetation was primarily lightly wooded eucalypt, dominated by tropical tall grasses including Mitchell grass [19]. Trial 2 was 9 km east from the Trial 1 paddock on the same property, in a 7 km\({}^{2}\) paddock with similar vegetation and terrain as Trial 1. Trial 2 paddock had one water point and was smaller to gain easier access to calving cows. Trial 1. Animals were unstered in April 2018 with a helicopter (R-22, Robinson Helicopter Company, Torrance, CA, USA) as per routine mustering procedures in the region. Animals were brought into a set of holding yards to be processed for the biannual husbandry checks, pregnancy testing via rectal palpation and vaccinations. A total of 926 cows were pregnancy tested by a veterinarian. Trial cows were selected from this group based on the gestation stage. Cows due to calve before the 1 st of September were eligible to be included in the trial, resulting in 157 pregnant cows and eight bulls. Trial cows were between 2 and 4 years of age, with 80% of the herd Brahman cows (_Bos Indicus_), with the remainder of the animals being composite or Shorthorn crosses. Bulls in the trial were a mix of Brahman (2), Composite (2) and Boran (4). All females were pregnancy tested, inspected for their health, provided with a management visual tag and electronic ear tag, weighed, and breed, body condition score and year of branding were recorded. Twenty (\(n=20\)) cows were selected and fitted with GNSS collars containing GPS, temperature, relative humidity, UHF, VHF, two-way Iridium satellite communication and accelerometers. There were 10 GNSS collars with corresponding VIT devices for Brand 1 (Iridium Track M 3D collar, Lotek Wireless, Havelock North, NZ) and 10 GNSS collars with corresponding VIT devices for Brand 2 (Vertex Plus Iridium collar, Vectronic Aerospace GmbH, Berlin, BB, Germany). The VITs were UHF ID coded and communicated with collars via UHF. Each of the collared cows had a VIT inserted by a veterinarian using an applicator, following the same insertion methods as the controlled internal drug release (CIDR) hormonal implant [20]. The cows were between 6 and 8.5 months of pregnancy according to rectal palpation. Animals were handled and subjected to standard commercial conditions for the duration of the trial. Animals were unstered back to the yards with a helicopter in October for calves to be weaned and for husbandry checks. Trial 2. Trial animals were unstered in April 2019 following the same method and procedures as Trial 1. Ten (\(n=10\)) cows at 5 months gestation were fitted with Brand 2 collars and corresponding Brand 2 VIT and allowed to graze in a smaller paddock (7 km\({}^{2}\)) where they were monitored closely over the calving period. Nine of the 10 cows with a VIT had not had an implant before. Cows were due to calve in August and were monitored at least 3 times a day during the calving period. Animals had access to one water point in the paddock. During calving, cows were monitored once a day in Trial 1 and 3 times a day in Trial 2.