# Genetic architecture of backfat thickness in laying hens: implications for extended egg production

## TL;DR

This study explores backfat thickness in laying hens as a potential trait for breeding to extend egg production while maintaining health.

## Contribution

The study identifies backfat thickness as a heritable trait with genetic correlations to production and health traits in laying hens.

## Key findings

- Backfat thickness is highly heritable and genetically correlated with body weight and abdominal fat.
- A major gene influencing backfat thickness was detected on chromosome 9.
- There is a trade-off between egg production and backfat thickness that may limit extended laying periods.

## Abstract

To extend the egg production period of laying hens, it is important to understand the energy trade-offs during the late laying period (after 80 weeks of age) to avoid decreasing egg production or compromising health through inadequate body reserves. Although egg production is routinely recorded, fatness is not. In this study, we were interested in backfat thickness as a new selection criterion for fatness in laying hens, analyzing its genetic architecture and relationships with energy-related traits. Data were collected from a commercial pure line of Rhode Island Red layers. Hens were phenotyped at 90 weeks of age for backfat thickness by ultrasound, feed intake, body weight, abdominal fat pad weight and liver weight, and throughout the period (70–90 weeks) for total egg mass laid. Hens were genotyped on a 60k SNP chip. Genetic parameters were estimated using a multivariate animal model. Segregation analysis was used to detect major gene influencing backfat thickness, and a genome-wide association study was performed.

Backfat thickness was highly heritable (h2 = 0.47) and showed a moderate positive genetic correlation with abdominal fat pad weight and with body weight. It showed moderately negative genetic correlation with total egg mass and a very weak positive genetic correlation with feed intake. Feed intake and total egg mass had a moderate positive genetic correlation. A major gene influencing backfat thickness was detected. A QTL was localized on chromosome 9.

Backfat thickness showed potential as a selection criterion to maintain fatness in laying hens for extending the laying period. Using body weight as a selection criterion to manage fatness is probably not optimal because its genetic correlation with fatness was only moderate. In addition, the trade-off between egg production and backfat thickness may be a limiting factor for extending the laying cycle. Inclusion of backfat thickness at 90 weeks in the selection index also requires estimation of genetic correlations with all other criteria, including those at early stages of production. In addition, the effect of the identifed major gene on backfat thickness and other traits must be detailed before inclusion of this new trait in breeding programs.

The online version contains supplementary material available at 10.1186/s12711-026-01036-x.