Abstract: [Objective] This study aimed to investigate the effects of heat stress on production performance, milk composition,and blood biochemical indicators of dairy cows in different lactation stages in the Linyi region. [Methods] A total of 24 healthy Holstein cows were selected and divided into three groups according to days in milk: early lactation （≤60 d）, mid-lactation （61~150 d）,and late lactation （≥151 d）. The experiment was conducted from May to August 2024. Milk yield,respiratory rate,rectal temperature,milk composition,and blood indicators （HSP70,prolactin,leptin） were monitored during non-heat stress and heat stress periods. [Results] Heat stress significantly reduced milk yield in all lactation stages （P< 0.05） and significantly increased respiratory rate and rectal temperature. The lactose and solid content increased significantly in early and mid-lactation cows （P< 0.05）,while milk fat rate and milk protein rate showed no significant changes. Heat stress also significantly up-regulated HSP70 expression and reduced prolactin levels in early and mid-lactation cows （P< 0.05）,with no significant change in leptin levels. [Conclusion] Heat stress significantly affects the production performance,milk quality,and blood indicators of dairy cows,with early and mid-lactation cows being more susceptible. The results provide a theoretical basis for developing regional strategies for the prevention and control of heat stress in dairy cows.

Key words: heat stress, lactation stage, milk yield, milk composition, blood indicators, Linyi region