Figure 1.

Regression plots of the association between age (Days) and rectal temperatures (RTs) of broilers raised under thermoneutral conditions (A) and heat stress (B). Polynomial regression analysis revealed a significant quadratic trend for both thermoneutral (y =  − 0.004x² + 0.16x + 40.2, R² = 0.65, P < 0.001) and heat stress (y = 0.063x² − 0.54x + 43.9, R² = 0.41, P = 0.036) conditions. The treatments are as follows: CON chicks hatched from control eggs without in ovo injection and incubated at standard temperature, CON + HS chicks hatched from control eggs without in ovo injection, incubated at standard temperature but exposed to HS, G10 + HS chicks hatched from eggs injected at embryonic day (ED) 17.5 with 0.6 mL of 10% GABA dissolved in distilled water and exposed to HS, TM + HS chicks hatched from thermally manipulated eggs exposed to 39.6 °C for 6 h daily from ED 10 to 18 and exposed to HS, G10 + TM + HS chicks hatched from eggs that received both previous treatments during incubation and exposed to HS.