Veterinary Science - Theses

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    Immune responsiveness as a predictor of health and welfare of dairy cattle in the Australian pasture-based production system
    Aleri, Joshua ( 2016)
    The management of dairy cattle health and welfare during the periparturient period remains a major challenge worldwide. Research in the intensive Canadian dairy production system has demonstrated that the health and welfare of dairy cattle can be improved when selection and breeding of animals is partly based on their inherent ability to fight and resist diseases, through what is referred to as immune competence testing. Dairy cattle ranked with higher immune responses had lower incidences of mastitis, milk fever, metritis, retained placenta, abomasal displacement, ketosis, and also had better production and reproductive performances compared to their counterparts with lower immune responses. Exposure to environmental pathogens is likely to differ between intensive and extensive dairy management systems. Therefore, in order to evaluate a similar approach in the predominantly pasture-based production system of Australia, we assessed immune responses in Holstein-Friesian heifers in typical Australian systems. Thus, a series of studies were conducted, including the development of a suitable protocol to assess immune responses under Australian conditions and testing the associations between immune responses in Holstein-Friesian heifers and specific health and production parameters (stress responsiveness, resistance to internal parasites, growth rate, individual somatic cell counts, milk yield, fat and protein content). A total of four experiments were conducted in this study using animals from commercial dairy farms in Victoria Australia reared under pasture-based systems, animals from the Department of Economic Development Jobs, Transport and Resources in Ellinbank, Victoria, Australia (DEDJTR) research centre and a group of bull calves obtained from commercial farms within New South Wales, Australia that were raised and restricted within the CSIRO quarantine facility located in Armidale, New South Wales, Australia. A commercial vaccine was used to induce measurable antibody and cell-mediated adaptive immune responses and assess general immune responsiveness in all the animals used for this study, whereas the HEWL+Candida albicans test antigens were additionally used on the bull calves restricted at the quarantine facility at CSIRO. The initial study determined the repeatability of immune responses in 100 Holstein-Friesian heifers reared in a pasture-based production system when at 5-6 months of age and as yearlings (12-13 months of age). Furthermore, the data were examined for associations between immune responses and stress responsiveness, resistance to internal parasites and average daily weight gains. Repeatability was moderate to high for antibody-mediated immune responses but poor for cell-mediated immune responses. In this study we also established that heifers with higher antibody-mediated immune responses also had higher weight gains than their lower antibody-mediated immune responding counterparts. In addition, significant variability of stress responsiveness was observed within the animal groups. The main experiment in this study was a prospective cohort study that assessed adaptive immune responsiveness in first lactation Holstein-Friesian heifers reared under a pasture-based production system, the relationships between immune responses and stress responsiveness, resistance to internal parasites, udder health and production performance were investigated. The results showed that stress and antibody-mediated immune responses were negatively correlated and animals with low antibody- and cell-mediated immune responses showed higher stress responsiveness compared to their corresponding counterparts. A short prospective study was conducted in adult Holstein-Friesian cows selected with either extremely high or extremely low feed conversion efficiency with the objective of establishing the relationship between feed conversion efficiency, immune responses and stress responsiveness. No significant differences in antibody or cell-mediated immune responsiveness were observed between the extreme high and low feed conversion efficiency phenotypes. However, results suggested that plasma cortisol concentrations trended higher in the low feed conversion efficiency phenotype cows as compared to their high feed conversion efficiency counterparts 48 hours post-yarding and handling. A significant negative correlation was observed between antibody-mediated immune responsiveness and stress responsiveness. In the last experiment conducted at CSIRO quarantine facilities, we assessed the correlations of immune responses measured using the commercial vaccine protocol to those obtained using the specific HEWL+Candida albicans test antigens used in Canada. In this study, the results obtained from the two protocols showed a similar trend. In conclusion, the commercial vaccine protocol developed within these studies for the assessment of immune responses within the Australian pasture-based production systems was effective and reliable and could be used to consistently identify low immune responders as early as 5 months within the production platforms. Furthermore, high immune responders had better stress coping mechanisms and increased weight gains. However, the relationship between immune responsiveness and resistance to internal parasites was inconclusive.