Page 14 - Marieke Poppe
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1 General introduction

                only production traits, which carried on until the ‘90s in most countries. Focus was
                on milk yield in dairy cattle (Miglior et al., 2005), growth and carcass traits in pigs
                (Rauw et al., 1998; Knap and Rauw, 2009), and either meat or egg production in
                poultry (Siegel et al., 2009). This selection was successful, because together with
                improved  feed  quality  and  management,  it  resulted  in  a  yearly  increase  in  milk
                yield of around 100-125 kg per lactation (Veerkamp et al., 2009), a yearly increase
                in growth rate in Dutch pigs of about 5 grams per day (Rauw et al., 1998), and a
                yearly decrease in age at a given body weight in broilers of about 1 day (Rauw et
                al., 1998).
                   Although  selection  on  production  was  successful,  negative  consequences  of
                increased productivity on other life functions were observed through unfavorable
                genetic  correlations.  In  dairy  cattle,  negative  trends  were  reported  for  fertility
                (Rauw et al., 1998; Veerkamp et al., 2009) and health (Rauw et al., 1998; Miglior et
                al.,  2005).  In  pigs,  a  reduced  ability  to  deal  with  suboptimum  conditions  was
                reported (Knap and Rauw, 2009). In poultry, decreased fertility and increased heart
                failure, ascites, bone disorders, and mortality were observed (Rauw et al., 1998). To
                mitigate the negative effects of selection for production on other life functions and
                to improve ‘robustness’ (Veerkamp et al., 2013), traits related to fertility, health,
                and survival were added to the breeding goal (Miglior et al., 2005, 2017; Neeteson-
                van Nieuwenhoven et al., 2013; Phocas et al., 2016). Increasing emphasis has been
                placed  on  these  new  traits  at  the  expense  of  production  ever  since,  and  the
                negative trends observed for these health and fertility traits have since then been
                reversed to a certain extent (Siegel et al., 2009; Kapell et al., 2012; Neeteson-van
                Nieuwenhoven et al., 2013; Miglior et al., 2017; Brito et al., 2021).

                1.3 The desire for genetic selection for resilience
                   Until now, the selection on health traits has focused on specific disorders that
                occur often and have large economic impact (Phocas et al., 2016), such as clinical
                mastitis (Heringstad et al., 2000; Nielsen  et al., 2000; de Haas et al., 2008; CRV,
                2020c) and claw disorders (Alkhoder et al., 2010; Gernand et al., 2013; CRV, 2020d;
                Egger-Danner  and  Heringstad,  2020).  However,  there  is  desire  (at  least  in
                developed countries) for less controlled, more natural animal production systems,
                where multiple pathogens and other environmental disturbances may act (Phocas
                et  al.,  2016).  In  addition,  even  more  environmental  disturbances,  such  as  heat
                waves, are expected in future due to climate change (Maracchi et al., 2005; Sejian
                et  al.,  2015;  Ge  et  al.,  2016;  Brito  et  al.,  2021).  Furthermore,  herd  sizes  are
                increasing  and  more  animals  have  to  be  managed  per  person  (Fraser,  2008;  ten
                Napel et al., 2009). Under such conditions, a more holistic approach is desired to

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