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Breeding methods are currently undergoing important changes, whether they be the elimination of cage farming, an objective of the European Union for 2027, systematically providing animals access to the outdoors or the more parsimonious use of medicines. These changes, intended to improve the welfare of farm animals and conforming to the "One health" principle, will necessarily implicate that animal environments will be less controlled and more subject to disturbances such as temperature variations or exposition to infectious agents. Within such a context, selection of more resilient animals, able to cope with these environmental disturbances and to recover quickly afterwards, appears more and more a necessity. However, until now, breeding programs for laying hens have not integrated an improvement of resilience.

A study on data collected from 60 000 laying hens

Egg-laying data are recorded daily for each hen throughout her life. A drop in the daily rythm of egg laying is often the sign of a problem in an animal when it is faced with environmental disturbances. That is why scientists analysed egg-laying data from 60 000 laying hens belonging to two lines used in selection programs (White Leghorn and Rhode Island Red) over a period of 58 weeks. They also analyzed the data from 17 000 crossbred hens from both of these strains used in commercial breeding. They first estimated the expected rate of egg-laying without any disturbances for each hen, which corresponds to the average number of eggs laid every week and by each hen. Then they calculated for each hen, the differences between the real and expected egg-laying rates.

Three indicators for resilience are in part of genetic origin

Three resilience indicators were calculated from the statistical analysis of individual deviations from expected egg production:

• An indicator for more or less sensitivity to disturbances (asymmetric). In the hen that is sensitive to disturbances, drops in laying are more frequent than in resilient hens, which is translated by an important asymetry due to deviations from the expected egg produciton.
• An indicator of recuperation speed after disturbances (autocorrelation). In hens that have difficulty recuperating, a return to the normal level of laying is slow. So, the deviations of egg production do not evolve much between two consecutive weeks, indicating a high autocorrelation.
• A general indicator of resilience (variance), which includes hen sensitivity to disturbances and its capacity to recuperate. A low variance corresponds to a resilient hen, with fewer deviations from the expected egg-laying curve and deviations of lesser amplitude.
• A good resilience corresponds to an aymmetry close to zero, and to low autocorrelations and variances. A scientific analysis shows that approximately 10% interindividual variability of these three indicators are of genetic and heritable origin. It is therefore possible to include resilience improvement in selection programs of laying hens. In addition, by preventing a fall in egg-laying, selection on resilience will indirectly increase egg production.

Overall, the two pure lines show a good resilience, which can still be improved by selection in each line. However, scientists show that genetic improvement of resilience in pure lines will only partially improve resilience in crossbred hens from these lines. This is important information since it is these crossbred hens that are raised for commercial egg production. This dificulty can be by-passed by integrating measures of resilience on crossbred lines, in addition to those on purebred lines in selection programs.

These original results show that it is possible to measure resilience, a complex trait, via three indicators that are relatively easy to calculate. They lead to new perspectives for selection of more resilient laying hens in a context of climate change and the transition to cage-free and more welfare friendly breeding.

REFERENCE

Nicolas Bedere, Tom V.L. Berghof, Katrijn Peeters, Marie-Helene Pinard-Van der Laan, Jeroen Visscher, Ingrid David, Han A. Mulder, Using egg production longitudinal recording to study the genetic background of resilience in purebred and crossbred laying hens, Genetics Selection Evolution 54, 26 (2022). https://doi.org/10.1186/s12711-022-00716-8