Abstract
Genetic variability of mastitis resistance is well established in dairy cattle. Many studies focused on polygenic variation of the trait, by estimating heritabilities and genetic correlation among phenotypic traits related to mastitis such as somatic cell counts and clinical cases. The role of Major Histocompatibility Complex in the susceptibility or resistance to intrammamary infection is also well documented. Finally, development from molecular genome mapping led to accumulating information of quantitative trait loci (QTL) related to mastitis resistance and better understanding of the genetic determinism of the trait. From economic and genetic analyses, and according to welfare and food safety considerations and to breeders and consumer's concern, there is more and more evidence that mastitis should be included in breeding objective of dairy cattle breeds. Many countries have implemented selection for mastitis resistance based on linear decrease of somatic cell counts. Given biological questioning, potential unfavourable consequences for very low cell counts cows are regularly investigated. Improvement of selection accuracy for mastitis resistance is ongoing and includes: advances in modelling, optimal combination of mastitis related traits and associated predictors, such as udder morphology, definition of global breeding objective including production and functional traits, and inclusion of molecular information that is now available from QTL experiments.
Highlights
Mastitis resistance is a complex trait, depending on a genetic component and on physiological and environmental factors, including infection pressure
Most genetic studies focused on milk somatic cell counts (SCC) and clinical mastitis as phenotypic measure to predict the bacterial status of udders [27, 51]
Genetics of mastitis resistance in dairy cattle has been studied for a long period
Summary
Mastitis resistance is a complex trait, depending on a genetic component and on physiological and environmental factors, including infection pressure. In many countries, frequency of clinical mastitis increased over time, at least in Holstein population. This trend resulted from the efficient selection based on productivity and from the genetic antagonism between milk production and mastitis resistance [90]. Many years after the Scandinavian countries, most breeding schemes were reoriented in the 90’s to efficiently include mastitis resistance in the objective and stop any unfavourable trend. We describe how mastitis resistance is accounted for in dairy cattle selection, according to the conventional stepwise procedure: definition of the breeding objective, choice of selection criteria, estimation of breeding values, and inclusion in breeding programs
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