Page 13 - Marieke Poppe
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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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