Seasonal heat stress alters the milk yield, antioxidative levels and serum metabolites in primiparous and 3 multiparous dairy cows

Abstract This study aimed to explore the seasonal heat stress affects the milk yield, antioxidative levels and serum metabolites in primiparous and multiparous dairy cows during early lactation. A total of two hundred dairy cows were selected according to their calving months (June, temperature humidity index (THI) =66.72; July, THI=70.30; August, THI=69.32; September, THI=67.20; October, THI=59.45). Blood samples were collected on day 0, 21, 50, 80, 100 after calving for serum oxidative status analysis and milk yield was recorded every day. The lower average daily milk yield in cows that calved in June and July (P<0.05), and average daily milk yield of multiparous cows was higher than that of primiparous cows that calved in the same month (P<0.05), suggesting that seasonal (June) heat stress negatively affected milk yield in both primiparous and multiparous cows at early lactation. Besides, 15 and 11 serum metabolites were changed in heat stress (average THI = 70.30) group compared with non heat stress (average THI = 59.45) group in primiparous cows and multiparous cows, respectively. These metabolites were mainly involved in the pathways of aminoacyl-tRNA biosynthesis, glyoxylate and dicarboxylate metabolism, and the metabolism of glycine, serine and threonine. These data suggested that heat stress negatively affected the milk yield and thus caused the the elevation of 2 the serum oxidative and antioxidative index Metabolic biomarkers associated with the heat stress 29 in serum were found 0 which providing the basement of evaluating indicator between heat stress and non heat stress groups in primiparous and multiparous cows.