Optimisation of genetic evaluations for longevity in Holstein dairy cattle through special consideration of health traits, SNP marker data and genotype by environment interactions




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From an economic and welfare perspective, the increasing importance of longevity in the dairy cattle sector has resulted in the inclusion of longevity in many national animal breeding programs. Ideally, reliable estimates of longevity should be made early in life. However, phenotypic records of longevity are typically unavailable before the end of a cow´s life. Thus, it is important to identify indicator traits that can be measured in the early stages of a cow´s life in order to improve longevity. Furthermore, other traits used in breeding goals may have an impact on longevity, which could lead to a limited selection gain in longevity. Additionally, as a result of genotype by environment interactions (G×E), the same genotypes may respond differently in various environments, leading to a bias in estimated breeding values and restricted selection responses. Therefore, the estimation of G×E in different production systems (e.g., conventional and organic farms) is imperative for using in organic farms bulls proved in conventional production system. The main objectives of the present thesis were to (1) estimate genetic parameters for longevity traits based on a variety of definitions and study the genetic and phenotypic impact of health traits on longevity, (2) evaluate different statistical models on heritabilities of longevity and (3) demonstrate G×E interactions in organic and conventional dairy production systems with regard to longevity and health traits in German Holstein cattle.In summary, the low heritabilities for longevity and the negligible contributions of significant SNP associated with longevity on culling risk ratios indicate the polygenetic nature of longevity. Furthermore, due to the favorable genetic correlations between longevity and health disorders, the inclusion of cattle health traits in breeding programs can accelerate the genetic improvement of longevity. However, more data from phenotyped and genotyped cattle at organic farms must be collected to improve the accuracy of genetic correlations between same traits from organic and conventional farms.




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