Effects of in ovo taurine administration on cyclic heat stress in broiler chickens

High temperature is a known abiotic stressor in broiler chickens, causing oxidative damage and altering gene expression. The present study was conducted to study the role of in ovo feeding of taurine against heat-induced damage in the broiler chickens. It was hypothesized that pre-hatch supplementation with taurine induces epigenetic changes such as DNA methylation and demethylation, which could help develop resistance to heat stress (HS) at later stages of life. For this, at 17.5 days of incubation, 360 fertile eggs from 37-week-old Arbor Acre breeder hens were divided into four groups: injected with distilled water (0TAU) × 2, and others injected with taurine at 1 %, 3 %, or 5 % concentrations (1TAU, 3TAU, 5TAU). For the in ovo feeding, a 23-gauge needle was used to deposit 0.6mL of solution into the amniotic sac. During rearing days 29 to 34, broiler chickens were exposed to a cyclic heat stress (HS, 31 ± 1 °C, 8 hours) or kept at a thermoneutral temperature (TN) zone (21 ± 1 °C). Hence, the treatment groups were: (i) 0TAU-TN, (ii) 0TAU-HS, (iii) 1TAU-HS, (iv) 3TAU-HS, and (v) 5TAU-HS. While the organ indices, average daily feed intake (ADFI) and feed conversion ratio (FCR) did not differ significantly, in ovo taurine linearly increased average daily gain (ADG) during the heat stress (HS) period (p = 0.032). The 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity% (DPPH-RSA%) in plasma showed a linear increase (p = 0.001) with taurine doses. Among the studied plasma metabolites, only alanine transaminase (ALT) was significantly affected, being lower in 1TAU-HS and 3TAU-HS compared to 5TAU-HS (p = 0.022). Individual gene expressions showed no significant variation across treatments. However, a planned contrast revealed upregulation of DNA methylation genes in the 5TAU-HS group compared to the 0TAU-TN group (p = 0.030). Strong positive correlations were observed among DNA methylation, demethylation, and NADPH oxidase (NOX) -related genes, suggesting coordinated regulation. Negative correlations between MDA and antioxidant enzymes indicated oxidative stress-related damage under HS. Hence, taurine linearly improved ADG under HS. While it did not significantly influence individual gene expression, 5TAU upregulated the overall DNA-methylation-related genes, suggesting a possible long-term adaptive response under HS.