Characterisation of contract heifer rearing in Ireland and associated biosecurity and animal disease risks and outcomes

Recent rapid expansion of Ireland’s dairy industry has resulted in increased uptake of contract-rearing services by dairy farmers. Inherent features of this enterprise such as animal movement and commingling of animals from multiple sources are risk factors for disease transmission that may limit the future success of the enterprise. Little published data are available relating to the biosecurity risks associated with contract-rearing practices in Ireland or internationally. As a result, the aim of this thesis was to elucidate the animal health risks associated with contract heifer rearing. To achieve this, three cohorts of Irish farmers were enrolled in a 3-year longitudinal study: farmers sending their heifers to a contract-rearing unit (source dairy farmers, SDFs), the farmers providing contract-rearing services and dairy farmers rearing their own heifers (control farmers, CFs). Firstly, farm management and biosecurity measures implemented by SDFs and CFs were characterised by questionnaire surveys. Results indicated that implementation of biosecurity measures was comparable across farm types. The second objective of this research was to compare the herd prevalence of a range of endemic pathogens on SDFs and CFs using bulk tank milk analysis. No significant differences in infectious disease status between farm types were observed. The third objective of this research was to compare the health status of contract and home-reared heifers. Health score data were collected for approximately 5,500 heifers on four occasions. Results indicate that the health status of home- and contract-reared heifers was comparable. Finally, the growth and reproductive performance of heifers were assessed. Study heifers were weighed on four occasions over an 18-month period and their average daily gain was calculated. Reproductive performance data were also gathered. Results showed that growth rates and reproductive performance of heifers were comparable across home-and contract-reared heifers. The consistent, cumulative findings of this research indicated that contract-rearing of replacement heifers did not negatively impact on the health, growth and performance of heifers or the infectious disease status of the source dairy herd. These novel findings have major implications for the future of contract-rearing both in Ireland and internationally as they provide evidence for the first time to dairy farmers and their service providers (e.g., veterinarians and agricultural advisers) that this enterprise can be a viable alternative to home-rearing of heifers in an era of dairy herd expansion.