Body-weight variation as a potential indirect indicator of abdominal fat weight with relevance to laying rate in 90-week-old laying hens.

In livestock species, efficient regulation of body lipid reserves contributes to prolonged reproductive performance, as shown in dairy cattle and more recently in laying hens through abdominal fat reserves. Controlling abdominal fat weight (AFW) at 90 weeks may improve metabolic efficiency and late egg production. However, AFW measurement requires slaughter and dissection, preventing routine use. This study therefore evaluated the genetic relationship between adult body-weight variation (∆BW) and AFW, and the potential of ∆BW as a non-invasive indirect indicator of AFW in aging hens. Genetic parameters were estimated for ∆BW between 30 and 70 weeks (∆BW3070), 30 and 90 weeks (∆BW3090), and 70 and 90 weeks (∆BW7090). Strong genetic correlations were observed between AFW and both ∆BW3070 (0.76) and ∆BW3090 (0.78), whereas the estimate for ∆BW7090 was lower and less precise (0.60). ∆BW3090 was positively genetically correlated with laying rate at 90 weeks of age (LR, 0.24), similarly to AFW, whereas ∆BW3070 was not (0.03). ∆BW7090 showed a higher positive correlation with LR (0.57), suggesting that late ∆BW may contribute more to the genetic relationship with laying persistence. These results support the evaluation of ∆BW3070 and ∆BW3090 as complementary non-invasive indicators of abdominal adiposity. ∆BW3070 can be obtained 20 weeks before direct AFW measurement, whereas ∆BW3090 shows a stronger genetic relationship with late LR. Their predictive ability and usefulness for selection nevertheless remain to be validated. Genome-wide association studies identified three QTLs for ∆BW3070 and two for ∆BW3090, none of which overlapped with the AFW QTLs previously detected in this population. LD-guided refinement reduced the size of some QTL regions by more than 40 %, narrowing one region to only four candidate genes. An eQTL approach also highlighted four additional candidate genes likely regulated by common variants, but these associations and colocalizations do not establish causal effects. Overall, ∆BW3070 and ∆BW3090 are potential non-invasive indicators of AFW in this population. However, external validation and evaluation of predictive ability and correlated selection responses are required before implementation.