Abstract
Summer high temperatures (27-38 °C, more than 7 days) readily induce heat stress in late-laying broiler breeders, which impedes offspring growth and development. This study aimed to provide a scientific basis for improving the offspring development of late-laying broiler breeders during summer. Six hundred fertilized eggs from 50-week-old LiFeng broiler breeders were divided into five treatment groups (non-injected, NaCl-injected, 6 mg/egg L-arginine-injected, 3.5 mg/egg L-methionine-injected, 8.4 mg/egg L-leucine-injected), with six replicates per group and 20 eggs per replicate. Embryos were incubated for 21 days and chicks raised for 21 days post-hatch. Methionine injection significantly enhanced hatchability (+5.8%), increased daily chick weight gain (+8.8%), reduced serum urea nitrogen (-53.13%), decreased inflammatory cytokine levels, elevated antioxidant enzyme activities, lowered malondialdehyde content (-47.99%), and suppressed expression of inflammatory and apoptotic pathway genes. The comprehensive effect of methionine was the best among the three amino acids when injected into chicken embryos. Methionine promoted protein synthesis, enhanced antioxidant and anti-inflammatory capacities, and consequently improved chick growth performance.