Dietary L-Tyrosine Sustains Egg Production and Quality by Modulating Promoter CpG Methylation, Gene Expression, and Hormone Profiles in Reproductive Signalling Pathways of Aged Laying Hens

Abstract

Managing prolactin (PRL)-mediated reproductive suppression is critical for maintaining egg production in commercial laying hens. L-tyrosine (TYR), a precursor of dopamine (DA), may counteract age-related reproductive decline by suppressing PRL, thereby sustaining productivity and egg quality in late-phase laying hens. The present study evaluated the effects of dietary L-tyrosine supplementation on laying performance, egg quality, reproductive hormone profiles, gene expression, and promoter CpG methylation in aged laying hens. A total of 180 Atak-S laying hens (71 weeks old) were randomly assigned to three dietary treatments, each with six replicates of 10 hens, and were fed a basal diet without L-tyrosine (control) or the basal diet supplemented with 0.5 g/kg (TYR0.5) or 1 g/kg (TYR1) L-tyrosine for 12 weeks. Hens in the TYR1 group exhibited higher hen-day egg production, improved feed conversion ratio, and significantly greater eggshell weight, shell thickness, Haugh unit, yolk index, and albumen index than the control group (p < 0.05). Dietary L-tyrosine increased plasma DA and serum estradiol while reducing serum PRL (p < 0.05). L-tyrosine also upregulated the expression of key dopaminergic and reproductive signalling genes, with corresponding promoter hypomethylation (p < 0.05), whereas PRL mRNA expression tended to decrease with concurrent promoter hypermethylation. A significant negative correlation was observed between target gene expression and promoter CpG methylation levels (p < 0.05). These findings indicate that dietary L-tyrosine modulates reproductive hormone profiles and molecular responses within reproductive signalling pathways, resulting in improved laying performance and egg quality. Therefore, L-tyrosine may represent a promising nutritional intervention to sustain reproductive function and productivity in late-phase laying hens.