BTB evaluations in beef cattle

The project has demonstrated the feasibility of undertaking genetic evaluations for bTB resistance. The amount of genetic variance observed in the trait warrants selection aimed at reducing incidence of disease by increasing host resistance. The impact of said selection on other beef traits is expected to be minimal.​

The project has delivered all the necessary information to integrate genetic evaluations for bTB resistance into AHDB's National Beef Evaluations. The full implementation of these genetic evaluations is underway.

The Problem:

Bovine tuberculosis (bTB) is a zoonotic disease endemic in the UK and other countries, and presents a significant challenge to the UK cattle sector. The Department for Environment, Food and Rural Affairs (DEFRA) lists bTB as one of the four most important livestock diseases globally, incurring annual costs of about £175 million in the UK.

Recent research conducted by the project team has led to the development of the first national genetic and genomic evaluation for bTB resistance in dairy breeds and the launch of TB Advantage by AHDB. The research confirmed the existence of significant genetic variation among individual animals for resistance to bTB infection, mainly inferred from the intradermal (skin) test and the presence of lesions and bacteriological tests following slaughter. Building on this know-how and experience, this project is aimed at the development of a national genetic evaluation for bTB resistance in beef cattle.

Aims and Objectives:

The overall aim of the project was to develop a system for routine national genetic evaluations of beef cattle for bTB resistance. In order to achieve the goal the project aims were to:

1. Develop a national database for the genetic evaluation based on bTB surveillance and pedigree records of beef cattle, combining data from the Animal and Plant Health Agency of DEFRA, the British Cattle Movement Service, individual breed societies, BASCO and the Edinburgh Genetic Evaluation Services
2. Define bTB resistance phenotypes for individual animals based on their health status; the latter will be defined from skin test results and post-mortem examination records
3. Estimate genetic parameters for bTB resistance in beef cattle using the phenotypes from step (2) after building and assessing relevant statistical models of analysis
4. Estimate breeding values (EBV) of individual animals for bTB resistance based on phenotypes from step (2) and genetic parameters from step (3)
5. Estimate the correlation of bTB EBVs with genetic evaluations for other traits in the breeding goal to assess the impact of selecting for enhanced resistance on beef traits
6. Develop genomic predictions and evaluations for bTB resistance based on results from step (4) and a validation process
7. Integrate all results into a system for routine genetic evaluations ensuring full automation of the process and including safeguards for the quality of the outcome

Approach:

The existing system of combining bovine Tuberculosis (bTB) surveillance records from the Animal and Plant Health Agency (APHA) with data from the British Cattle Movement Service (BCMS), breed societies and EGENES was used to develop a national bTB database for beef cattle genetic evaluations (Objective 1). The APHA data include intradermal (skin) test records and dates, and results of post-mortem examination for visible lesions and culture for the Mycobacterium bovis (the bTB pathogen). All beef breeds available in the APHA database were considered in this project. The BCMS data was used to complement those from APHA in identifying healthy contemporaries to positive skin test reactors prior to 2011 (i.e. the period when non-reactors were not recorded in the surveillance data). Pedigree data from EGENES and the breed societies provided information on the animals’ date of birth, sire and dam, and also the percentage of each breed. The latter was used to estimate levels of heterosis and recombination in the crosses, which was required for the genetic evaluation.

The health status of each animal with regards to bTB was determined based on results of the skin test and post-mortem examination; the method was previously applied on dairy data and is described in detail by Banos et al. (2016, J Dairy Sci). The derived health status constitutes the animal’s phenotype regarding resistance to bTB.

From these data genetic parameter estimates of the trait were determined.  Genetic evaluations (estimated breeding values – EBVs) calculated for each animal based on these genetic parameters.  These steps, together with further analysis, will be integrated into a routine genetic evaluation system. The process will be fully automated and monitored throughout with regular quality assurance checks.