Effects and Mechanisms Investigation of Heat Stress on Egg Yolk Quality in Huaixiang Chickens

SIMPLE SUMMARY: High ambient temperature is a significant factor contributing to economic losses in the poultry industry. Heat stress had adverse effects on the growth performance, reproductive performance, and egg production of poultry birds. However, the specific impact of heat stress on internal egg yolk quality parameters and the potential mechanisms of local chickens remain to be further studied. The purpose of this study was to explore the influences of heat exposure on internal egg yolk quality parameters and their possible mechanisms in Guangdong Province Xinyi Huaixiang chickens. Our results revealed that heat exposure (32 °C) for 6 weeks reduced yolk weight, yolk color, and egg weight. Additionally, we observed an upregulation of lipid metabolism-related genes, including SREBP-1c, ACACA, and FASN. These results provided valuable insights into the underlying mechanisms of egg yolk quality changes induced by heat stress. Understanding these mechanisms is crucial for mitigating the negative impact of heat stress on poultry production and improving the overall performance and well-being of poultry birds. ABSTRACT: The purpose of this study was to examine the effects of high temperature on internal egg yolk quality parameters and their possible mechanisms in Huaixiang chickens. This study consisted of two treatments, and each treatment had six replicates with six birds per cage. A total of seventy-two 26-week-old female Huaixiang chickens were randomly divided into a normal-temperature group (NT) and a high-temperature group (HT) for 6 weeks. And these hens were exposed to 25 ± 2 °C and 32 ± 2 °C, respectively. Their relative humidity was maintained at 55–65%. The results showed that the HT group significantly reduced yolk weight, yolk color, and egg weight compared to the NT group (p < 0.05). Heat stress caused vacuolar degeneration of the liver and reduced the absolute liver weight (p < 0.05). Both yolk triglyceride (TG) and liver TG in the HT group were significantly higher than in the NT group (p < 0.05). However, the liver total cholesterol (TC) level in the HT group was remarkably lower than that in the NT group (p < 0.05). Additionally, heat stress remarkably enhanced SREBP-1c, ACACA, and FASN lipid metabolism-related gene mRNA expression levels in Huaixiang chicken liver after 6 weeks of heat exposure (p < 0.05). Furthermore, the HT group had remarkably reduced total amino acid, Cys, and Tyr levels in the yolk when compared with the NT group in our experiment (p < 0.05). In conclusion, heat stress causes egg yolk quality reduction and abnormal lipid metabolism in Huaixiang chickens. These findings provided novel insights into the role of high temperature on egg yolk parameters and the underlying mechanisms in Chinese indigenous laying hens.