2.1. Research Sites, Animals, and Experimental Design

This observational cohort study was conducted using 30 maiden ewe flocks on 28 farms across Western Australia (n = 11 flocks), South Australia (n = 9 flocks) and Victoria (n = 10 flocks) between 2018 and 2020. Farms were located in medium (400–600 mm rainfall annually) to high (600–1000 mm rainfall annually) rainfall zones (Figure 1). The study was performed in consecutive years using different study flocks on two farms; one from Western Australia (2018 and 2019; Flocks 3 and 14) and one from Victoria (2019 and 2020; Flocks 19 and 27). All other farms were sampled in a single year.

Approximate location of each farm where maiden ewes were monitored in (A) Western Australia and (B) South Australia and Victoria. Data for average annual rainfall was sourced from Australian Government Bureau of Meteorology [21].

Farms were selected on convenience sampling with criteria for inclusion based on: having at least 200 maiden ewes available for the study, a capacity to monitor ewes and their progeny over the study period, and with a sheep genotype and management system that were generally representative of commercial sheep farms in the region. Some farms included in the study managed flocks of stud sheep, but management and stocking density were comparable to commercial sheep flocks in these regions.

Flocks of approximately 200 ewe lambs (n = 19 flocks; 7–10 months of age at joining) or Merino two-tooth ewes (n = 11 flocks; 18–20 months of age at joining) were monitored from joining to lamb marking on each farm. For farms with more than 200 maiden ewes, a subset of approximately 200 ewes were randomly selected from the larger cohort for inclusion in the study. Ewe lambs were of non-Merino breeds except for one research site (Flock 8) in Western Australia which joined Merino ewe lambs to Merino rams. Most sires were the same breed as the ewes in the study flock; however, White Suffolk rams were used in two flocks joining maiden Merino two-tooth ewes in South Australia (Flock 9 and 12). All rams were confirmed to be negative for Brucella ovis via serology prior to joining. Most ewes were naturally joined with a ratio of 50 ewes per ram and joined for an average of 38 days. Some flocks were separated into smaller mobs at joining to facilitate single-sire mating. All or some of the ewes in five flocks were artificially inseminated followed by a period of natural joining.

Each farm ran self-replacing flocks and sheep were managed as per standard farm practice including the monitoring of condition score to guide nutrition and grazing management. Seven of the thirty flocks had been vaccinated against Campylobacter spp. (Coopers Ovilis Campyvax^®, MSD Animal Health, Bendigo, VIC, Australia) according to the manufacturer’s instructions (Table S1). On most farms, the study flock was kept separate from other maiden and mature sheep. Some farms managed their single- and multiple-bearing maiden ewes together in a single mob whilst others separated their single- and multiple-bearing ewes prior to the start of lambing.