Page 14 - Marieke Poppe

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

12