摘要Heat stress can lead to decreased feed intake,apoptosis of mammary epithelial cells,and decreased milk yield and quality.Selenium is an important element in the composition of at least 25 selenoproteins.Hydroxy-selenomethionine(HMSeBA)is a novel organic selenium that has been shown to have a better deposition effect.However,whether HMSeBA alleviates damage to the mammary gland blood-milk barrier caused by heat stress and how this affects the performance of dairy cows remain largely unexplored.Therefore,32 healthy Holstein cows with similar gestation days(150.41±20.07 d),milk yield(36.15±3.02 kg)and parity(3.25±0.51)were selected and randomly divided into two total mixed rations with different selenium(Se)sources:sodium selenite(SSe)and HMSeBA.This study evaluated the outcomes of HMSeBA on antioxidant capacity,immunity,and blood-milk barrier damage in dairy cows during heat stress by collecting the samples of blood,rumen fluid and mammary gland biopsy.The experiment was conducted over 35 d,including a 5-day pre-feeding period and a 30-day experimental period.The tem-perature and humidity index(THI)were all above 80 throughout the experiment period.The results showed that HMSeBA decreased the respiratory rate(P＜0.001)and the content of inflammatory cytokines in the serum and increased the content of immune factors and antioxidant capacity(P＜0.05).In addition,HMSeBA reduced the expression of inflammatory cytokines and heat shock proteins in mammary gland(P＜0.05).Hematoxylin-eosin-stained pathological sections showed massive thickening of acinar walls and severe destruction of glandular structures in the SSe group,but the structure of the acinar mammary gland in the HMSeBA group was intact.Furthermore,HMSeBA promoted the expression of the phosphatidylinositol 3-kinase(PI3K,P＜0.001)/protein kinase B(AKT,P=0.011)/mammalian target of rapamycin(mTOR,P=0.008)pathway and improved the expression of zonula occludens-1(ZO-1,P=0.014)and occluding(OCLN,P=0.012)in the mammary gland,suggesting less damage caused by heat stress to the blood-milk barrier.Our results demonstrated that HMSeBA can improve the antioxidant capacity and immunity of dairy cows and the expression of tight junction proteins in mammary gland to help alleviate the blood-milk barrier damage by heat stress,which could reduce the damage of heat stress on milk yield.