Page 13 - Marieke Poppe

1 General introduction

1.1 Introduction
For many decades, focus has been on improving productivity of livestock, both
through genetic selection and optimization of the environment (Ge et al., 2016).
Despite more recent focus on improving resistance to individual diseases, society
still expresses concerns about animal welfare (Ge et al., 2016; Clay et al., 2020) and
farmers express a desire for problem-free cows (Elgersma et al., 2018; Berghof et
al., 2019b; Egger-Danner and Heringstad, 2020). Especially in the light of climate
change, which will likely lead to more environmental disturbances in the near
future (Boichard and Brochard, 2012; Urruty et al., 2016), we need animals that can
cope with disturbances well (Phocas et al., 2016). Such animals are called ‘resilient’,
and likely have good welfare and are easy to manage for the farmer (Colditz and
Hine, 2016).
There are several ways to improve resilience of animals, and one of them is
through genetic selection. Genetic selection is the selection of the genetically ‘best’
animals as parents for the next generation (Falconer and Mackay, 1996). In case of
resilience, the ‘best’ animals would be the most resilient animals. By selecting the
genetically most resilient animals as parents, the next generation will be on
average more resilient than the previous generation. However, as resilience is a
broad concept, it is difficult to quantify it and thus to identify which animals are
genetically most resilient.
When traits are difficult to measure on selection candidates, breeding programs
often make use of ‘indicator traits’ (Falconer and Mackay, 1996; Miglior et al.,
2017). Indicator traits are easily measurable traits that are heritable and are
genetically correlated with the trait of interest. Selection on the easily measurable
indicator trait will then result in improvement of the trait of interest as well. This
thesis focuses on the development of indicator traits for the complex trait
resilience from easily available, automatically measured longitudinal traits in dairy
cattle.
In this introduction, I will first describe the history of genetic selection, and how
this led to the current need for genetic selection for resilience. I will then focus on
previous research into selection for resilience. Finally, I will introduce new
opportunities for quantifying resilience, which lead to the main research topics of
this thesis.

1.2 History of genetic selection in livestock
To understand the need for genetic selection for more resilient livestock, it is
useful to first get an overview of the genetic selection in the past that led to the
livestock populations we have today. Genetic selection started off with selection on

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