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The reproductive performance of maiden (primiparous) ewes is an important component of overall sheep flock performance in Australia. However, the reproductive performance of maiden ewes that are mated for the first time either as ewe lambs at 7 to 10 months of age, or hoggets, at 18 to 20 months of age is poor and inconsistent. Lamb mortalities in the perinatal period are a major source of reproductive inefficiency and a major welfare concern, with approximately 20-30% of lambs born in Australia dying at or after birth. However, most industry data for lamb survival is based on mature or mixed age ewes and do not distinguish between in utero losses and those that occur at or after birth. Consequently, the timing, magnitude and nature of losses that occur between pregnancy scanning and lamb marking in maiden ewe flocks in Australia are not well described. Maiden ewes may be more susceptible to infectious diseases that cause abortion, stillbirths and the birth of weak lambs that are less likely to survive, as younger ewes are less likely to have developed immunocompetency to infection prior to pregnancy. In New Zealand, some studies have reported that infectious diseases are important contributors to the poor and inconsistent reproductive performance observed for maiden ewes, including an increased incidence of abortions in maiden ewe flocks. However, it is not clear if infectious diseases are an important contributor to foetal and lamb mortality for maiden ewes in Australia. The overall objectives of the research described in this thesis was to i) identify the most common causes of ovine abortion and stillbirths diagnosed in Australia, ii) quantify abortion and lamb mortalities that occur in maiden ewe flocks between pregnancy scanning and lamb marking and iii) determine the impact of endemic abortigenic pathogens including Toxoplasma gondii, Neospora caninum, Coxiella burnetii, Campylobacter fetus and Campylobacter jejuni on the reproductive performance of maiden ewe

At least 20% of lambs born will die in the first 3 days of life under extensive grazing conditions in southern Australia. Limited research has suggested that increasing the mob size or stocking rate of ewes at lambing may increase the risk of ewe-lamb separations and lamb mortality. Higher lambing densities are likely to exacerbate the attraction of foreign periparturient ewes to amniotic fluids and newborn lambs which may result in greater interference at lambing. This thesis aimed to quantify the impacts of mob size and stocking rate on the survival of lambs born on commercial farms. Furthermore, it aimed to determine whether poorer survival of lambs born at higher mob sizes was associated with greater interaction with foreign ewes and lambs at lambing. Survey data collected from 66 farms in south-eastern Australia indicated the survival of single- and twin-born lambs decreased by 1.4% and 3.5% per additional 100 ewes in the mob at lambing. Furthermore, it suggested that increasing stocking rate by 1 ewe/ha would reduce lamb survival by 0.7%, regardless of breed and birth type. Similar survey data collected from producers in New Zealand found the survival of Merino lambs decreased by 1.3% per additional 100 ewes in the mob at lambing, however there was no effect of stocking rate on lamb survival. Data collected from 60 on-farm research sites across southern Australia confirmed that higher mob sizes result in poorer lamb survival. In this study, the survival of twin-born lambs decreased by 2% per additional 100 ewes in the mob at lambing. However, stocking rate was not found to influence lamb survival suggesting that the number of ewes in the mob regardless of paddock size has a greater influence on lambing density. Detailed experimentation also suggests that the relationship between mob size and lamb survival may be influenced by feed-on-offer (FOO) at lambing. In one experiment, a higher mob size decreased the survival of twin-born lambs when FOO was below 400 kg

Cryptosporidium and Giardia are protozoan parasites that may cause gastrointestinal disease in a wide range of species, including humans and sheep, and impact profitability along the sheep meat supply chain. At present, sources of Cryptosporidium and Giardia infections on Australian sheep farms have not been described. This project aimed to determine whether contaminated drinking water supply, such as dams, may represent an important transmission source of Cryptosporidium and Giardia in extensively managed sheep, and whether the quantity of Cryptosporidium and Giardia (oo)cysts in dam water was greater during the winter months after high rainfall events. Water samples (n = 16) from 12 paddock dams and faecal samples (n = 274) from 11 mobs of sheep with access to the sampled dams were collected during autumn and winter from six commercial farms in south west Western Australia. Cryptosporidium oocysts and Giardia cysts were recovered from dam water samples using a calcium carbonate flocculation technique and genomic DNA was extracted from dam water and faecal samples with commercially available DNA extraction kits. Quantitative PCR (qPCR) was used to screen for the presence of Cryptosporidium spp. and Giardia spp. at the 18S ribosomal RNA (18S) locus and glutamate dehydrogenase (gdh) locus, respectively, and (oo)cyst quantitation was carried out using standards calibrated by droplet digital PCR (ddPCR) to estimate gene copy numbers. Conventional nested PCR was then used to amplify a longer fragment (>1 kb) of the 18S locus qPCR positive samples and the products were sequenced with Sanger sequencing to identify the Cryptosporidium species present in faecal and water samples. Cryptosporidium ubiquitum was further subtyped at the gp60 locus with nested PCR and Sanger sequencing. Conventional nested PCR that targeted the tpi locus of Giardia duodenalis assemblage A, B and E and Sanger sequencing were used on faecal samples that were qPCR positive for Giardia at the 18S lo