Introduction

Genetic improvement of laying hens is carried out in pyramid selection programmes. At the selection level, purebred animals are usually reared in individual cages, for all or part of their career, to allow monitoring of their individual performance. The selection criteria (Box 1) classically recorded are related to the animal's body weight, production (number of eggs laid), and egg qualities (egg weight and shape, eggshell strength and colour, white quality, and proportion of yolk; Beaumont et al., 2010). The best individuals, i.e., those with the best genetic values (Box 1) for a weighted combination of these criteria (index), are selected as future breeders. The offspring crossed between different lines are then disseminated on a large scale by the multiplication level, allowing the production level to benefit from genetic improvement by crossing pure lines (Coudurier, 2011).

In Europe, laying hens were mainly kept in collective cages at the production level. Since 2012, these cages have been called ‘enriched cages’ because they are equipped with nests, perches, and a scratching area. In recent years, a wide range of cage-free systems have also been developed to provide animals with living conditions that meet their needs. In these systems, hens move freely in a building with or without an aviary (several levels in the building; figure 1). Adjacent to the building, there is sometimes a covered and screened exercise area (winter garden; figure 1) that allows the hens to have access to natural light and outdoor weather conditions while being protected from the elements. Some buildings have outdoor access; these are called 'free range,' 'Label Rouge free range,' and 'organic free range' systems (figure 1). Systems without outdoor access are called floor systems (figure 1). Whether the hens have access to a winter garden or the outside, natural light is often supplemented by artificial light, especially when days are short. European legislation requires that the lighting programme should follow a 24-hour rhythm, with a continuous dark period of at least 8 hours, and a light intensity that allows hens to see and be seen clearly (Directive, 1999/74/EC).

In 2019, cage-free systems accounted for 52% of commercial laying hens reared in the European Union (European Commission, 2021), compared to 30% in 2009 and 8% in 1996 (ITAVI, 2019). For hens reared in cage-free systems, 36% had outdoor access in 2019 and 64% were reared on the floor (European Commission, 2021). Currently, there is no specific breeding programme for cage-free systems; hens reared in cage-free systems are derived from the same pure lines as hens reared in cages. Therefore, cage-free system rearing is based on selection indices established on animals reared in conventional systems.

Figure 1: Illustration of cage-free systems.

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In contrast to cage rearing, hens reared in cage-free systems are housed in a large group, must lay eggs in nests (figure 1), and sometimes have outdoor access. Electronic nests have been developed to measure new egg-laying traits in these environments (Box 1), related to the pre-laying behaviour, the nesting behaviour, and the laying rate. These traits now appear in the selection objectives of pure lines. The objective of this review is to present the current state of knowledge on egg-laying traits in cage-free systems, focusing mainly on the two most widely reared breeds of laying hens in the world: Rhode Island (RI; brown eggs) and White Leghorn (WL; white eggs).

1. Egg production

Egg production, defined as the number of eggs laid in a given period, or laying rate (number of eggs laid divided by the number of days in the period) is the major criterion considered in laying hen selection schemes. While egg production is influenced by the environment, mainly the photoperiod (Sauveur, 1996; England and Ruhnke, 2020), it is also dependent on a genetic component. Thus, the existence of genetic variability in egg production has contributed to the current level of production in commercial hens capable of laying more than 300 eggs per year.

After intensive selection on sexual maturity and egg production up to 55 weeks of age, breeders are now also interested in the persistency in lay, i.e., the ability of hens to have a longer laying career (Bain et al., 2016). Currently, laying hens are reared to about 72 weeks of age. Extending their laying career to 100 weeks, with a total production of almost 500 eggs per hen, would reduce the number of hens. In the UK, for example, a 100-week laying career would reduce the number of laying hens in the country by 2.5 million (Bain et al., 2016). In addition, a longer laying career would reduce the amount of feed and therefore the negative environmental effects, particularly by reducing the amount of nitrates produced for the same level of production (Bain et al., 2016). Thus, egg production of hens older than one year is often included in the selection objective of laying hen lines (persistency in lay and stability of egg qualities, such as egg weight and shape and eggshell strength and colour).

Measured in individual cages, the number of eggs laid from the age at the first egg to 90 weeks is moderately heritable, with heritability estimates (Box 1) of 0.26 and 0.34 for RI and WL, respectively. (Wolc et al., 2019). Many chromosomal regions (QTLs; Box 1) are involved in the genetic background of egg production. Twenty-one studies have detected 95 QTLs influencing the number of eggs laid and/or laying rate (Romé and Le Roy, 2016). In particular, the genes coding for three proteins involved in folliculogenesis – prolactin (chromosome 2), the follicle stimulating hormone receptor (chromosome 3), and the gonadotropin receptor (chromosome 22) – have been identified as influencing the number of laid eggs (Romé and Le Roy, 2016). Knowledge of the genetic background of egg production, acquired in cage systems, needs to be revisited in the context of less standardised cage-free systems.

1.1. Laying in nests

Egg production in nests, measured as the number of eggs laid in nests over a period or as the laying rate in nests (number of eggs laid in nests divided by the number of days in the period) is the proportion of egg production that is easily collected and valued by the farmer. The rapid increase in the number of laying hens kept in groups with access to collective nests has raised many questions about the ability of the animals to use the nests. In response, individual electronic nests using radio frequency identification (RFID) technology have been developed (Burel et al., 2002; Marx et al., 2002; Thurner et al., 2006). These nests are equipped with an antenna (figure 2). A transponder is attached to each hen, either in the neck (Burel et al., 2002), on a wing (Marx et al., 2002), or on a leg (Thurner et al., 2006). When a hen uses an electronic nest, her transponder is detected by the antenna of the nest. Its ID, the ID of the visited nest (location), the time of access, and the time of presence are then recorded. If the hen lays an egg, it rolls behind the nest and triggers a sensor that records the laying time. By sorting the eggs by laying time (order of passage of the sensor), each egg can be allocated to the hen that laid it. These nests thus make it possible to measure egg-laying and egg quality traits (via egg-hen awarding) at the individual scale. In this review, the results obtained with an electronic nest described by Thurner et al. (2006) will be presented because it is the only one that has been the subject of several publications thus far.

Figure 2. Principle of the individual electronic nest.

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Using electronic nests, Icken et al. (2008a) recorded the nesting behaviour of 272 RI hens reared in a floor pen with access to a winter garden. The number of eggs laid in nests measured over 12 periods of 28 days was found to be heritable, but heritability estimates varied with the age of the hens (between 0.05 at 9 months and 0.45 at 14 months). The estimated genetic correlation (Box 1) between the number of eggs laid in cages and the number of eggs laid in nests was also highly variable with hen age, ranging from strong positive values of between 0.56 and 0.97 during the first two months of egg-laying to lower estimates, between 0.22 and 0.44, at peak egg production (Icken et al., 2012). These genetic parameters, estimated on a small population, need to be clarified, but this original study may reveal the existence of genotype-environment interactions for egg production. The genetic correlation values different from 1 between cage and nest production may also be due to the significance of these two phenotypes; in the electronic nest, the hen's behaviour, i.e., her ability to go to the nest to lay eggs, is combined with her egg-laying capacity expressed in the cage. To improve egg production in cage-free systems, further studies are needed to improve knowledge of the nest laying behaviour traits of hens.

2. Pre-laying behaviour

Pre-laying behaviour begins one to two hours before oviposition (egg-laying) and is initially characterised by increased activity with greater hen agitation and vocalisation (Wood-Gush and Gilbert, 1969; Meijsser and Hughes, 1989; Sherwin and Nicol, 1993). The hen then chooses a laying site and settles in. Once settled, the hen adopts a 'sitting' position, which she will alternate with nest-building activities, such as scratching the ground or litter, turning around in the nest, and collecting litter, if available, before laying her egg. Under natural conditions, the behaviour of searching for an egg-laying site would allow the hen to find a place that is safe from predators, as observed by Duncan et al. (1978). In this study, hens kept the same nest to lay all the eggs in a clutch (a sequence of consecutive days of laying) but systematically changed it between laying clutches and seasons, probably to protect themselves from predators and parasites.

In breeding, hens would generally use the same nest to lay eggs. Wall et al. (2004) recorded the behaviour of 35 transponder-equipped WL kept in cages with two collective nests. Consistency in nest choice averaged 90%, with a value of 50% indicating that the hen had no preference and 100% that the hen would always lay in the same nest. Consistency values in nest choice ranged from 54 to 100%, indicating variability for this trait. These results should be confirmed in cage-free systems with more nests. Furthermore, hens have a gregarious behaviour when laying eggs. If given the choice, they will prefer to lay in occupied nests rather than in free nests (Appleby and McRae, 1986; Riber, 2010; Tahamtani et al., 2018). This behaviour can have negative effects in terms of animal welfare, potentially increasing stress levels and the risk of suffocation in nests but also in terms of production by increasing off-nest laying (hens laying in front of occupied nests) and the number of broken eggs. Selecting hens that are able to go and lay in different nests far from each other could reduce the negative effects of gregarious laying behaviour. In the literature, there is no work on this trait, although electronic nests can measure it (by recording the nest ID and therefore the location). However, results have been published on winter garden exploratory behaviour and stereotypical behaviour prior to egg-laying.

Exploratory behaviour in a winter garden was studied in RI hens. Measured between 9 and 16 months of age, the frequency of passages and length of stay in the winter garden are heritable with estimated values of 0.24 for these two traits, which are also strongly genetically correlated (+0.82; Icken et al., 2008a). However, unfavourable genetic correlations were observed between the average length of stay and the number of eggs laid in nests (–0.34; Icken et al., 2008a), as well as the eggshell colour of RI (–0.54; Icken et al., 2011). The offspring of RI hens that spend the most time in the winter garden lay fewer eggs and have lighter coloured eggs compared to the offspring of hens that spend less time in the winter garden.

Regarding pre-laying behaviour, it is mainly stereotypical behaviour that has been the subject of genetic studies. These behaviours, such as severe pre-laying agitation and standing during laying, show that the hen cannot express her natural pre-laying behaviours and are indicators of malaise. Heil et al. (1990) measured the duration of pre-laying agitation, escape attempts in the five to ten minutes before laying (number of times the hen pokes her head out of the cage), and the position (sitting or standing) during laying by video recording individual cages. They found that the duration of agitation and escape attempts were weakly heritable (0.06 and 0.04, respectively), but position was moderately heritable (0.33). The results already observed on exploratory behaviour in the winter garden and on stereotypical behaviour prior to laying support the interest in studying the ability of hens to lay in different and distant nests. This trait may one day allow indirect selection against negative gregarious behaviour for laying and off-nest laying. In addition to pre-laying behaviour, nesting behaviour could also be exploited in selection to reduce off-nest laying.

3. Nesting behaviour

Two traits can be used to describe the nest-laying behaviour of the hen: the time of entry in nests and laying duration, i.e., the time spent in the nest for laying.

4. The laying rhythm

The laying rate and the number of eggs laid are complex traits controlled by the hen's laying rhythm. On average, it takes 24 to 26 hours from ovulation of the yolk to laying the egg (Sauveur, 1988; Box 2). Ovulations can occur daily, resulting in one egg being laid per day. Eggs laid on consecutive days form a clutch (Box 3). Each clutch is separated by days without laying, called pause days.

Several traits can define the laying rhythm, such as the length and number of clutches, the time interval between laying, and the laying time. These traits can be of interest for increasing egg production, both in cage and cage-free systems. In cage-free systems, the laying rhythm must also be controlled, as it is linked to the time of access to the hen's nest, which can lead to off-nest laying (see section 3.1.). The development of electronic nests makes it possible to record these traits in cage-free systems, and their genetic parameters have already been estimated, both in individual cages and in cage-free systems.

Conclusion

The possible recording of pre-laying behaviour and nesting behaviour traits in cage-free systems offers prospects for selecting laying hens against off-nest laying. With heritability estimates often higher than egg production and favourable genetic correlations, the traits of the laying rhythm could also be used in selection, with the aim of increasing egg production per hen in cage-free systems. In contrast to the length and number of clutches, the time interval between laying and the laying time seems to be easier to measure with electronic nests because they do not require the total number of eggs laid, which also depends on the number of eggs laid off-nest (unknown). Selection based on the average laying time could nevertheless contribute to reducing the variability of laying time in the lines and thus the time of entry in nests, increasing competition and the risk of off-nest laying. Further studies, especially in cage-free systems, are needed to better understand the relationships between egg-laying traits and egg quality before considering their inclusion in breeding programmes. The relationships between these traits and resistance to pathogens, as well as the hen's microbiota, response to stress, and social behaviour, particularly aggressiveness and dominance, need to be studied to ensure that they are not degraded. The accuracy of heritability estimates and genetic correlations of egg-laying traits in cage-free systems needs to be improved with measurements on larger numbers. The extent of genotype-environment interactions between individual and collective nest systems and between various environments representative of cage-free systems (temperature, humidity, duration of lighting, outdoor access or not, feeding, etc.) must also be estimated. Finally, the consequences of crossbred individuals, i.e., on commercial hens resulting from selection, must be evaluated.

Acknowledgments

This article was first translated with www.DeepL.com/Translator and the authors used Proof-Reading Service (http://www.proof-reading-service.com) for English language editing.

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