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Heat stress adversely affects the metabolism and production of dairy cows, particularly during the transition period. The aim of this study was to evaluate energy metabolism in heat stressed dairy cows during the periparturient period. Twenty-four Simmental dairy cows were included in the study. The cows were assigned into two groups according to the season: the summer group (n = 12) and autumn group (n = 12). Physiological parameters (rectal temperature, respiratory and heart rate) and serum metabolic parameters (glucose, triglycerides, total cholesterol, high-density lipoprotein cholesterol (HDL-C), non-esterified fatty acids (NEFA) and beta-hydroxy butyrate (BHB) were measured at days -21, -7, 8, 16, 24, 32 and 40 relative to calving. There were no significant differences in glucose concentration between the investigated groups (P<0.05). Serum NEFA concentration was significantly higher (P<0.05) in the summer group before calving at day -7, while after calving it was significantly lower (P<0.05) at days 16 and 32 of lactation compared to the autumn group. The summer group had lower BHB concentrations after calving compared to the autumn one with statistical significance at days 8, 16 and 24 postpartum (P<0.05). Lipid parameters were also significantly lower (P<0.05) in the summer group after calving, particularly total cholesterol and HDL-C at days 16, 24, 32 and 40 postpartum. The results indicate that changes in energy metabolism in dairy cows under heat stress are directed to reduce NEFA oxidation, in order to limit metabolic heat production. These metabolic adaptations mostly occur after calving because dairy cows are more susceptible to heat stress in early lactation as they produce more metabolic heat during milk production. key words: heat stress; dairy cows; lipid mobilisation; non-esterified fatty acids; beta-hydroxy butyrate  DOI: 10.24099/vet.arhiv.0856  *Corresponding author: Romana Turk, PhD, Faculty of Veterinary Medicine, University of Zagreb, Heinzelova 55, Zagreb, Croatia, Phone: +385 1 2390 180; E-mail: rturk@vef.hr R. Turk et al.: Energy metabolism in heat stressed dairy cows Vet. arhiv 90 (1), 1-10, 2020 2 introduction Climate changes and global warming greatly affect the performance and production of dairy cows, and are having a significant impact on the dairy industry. Elevated ambient temperature is one of the recognized environmental factors contributing to disturbances in the general health and production of dairy cows. In these conditions, dairy cows produce a large amount of metabolic heat and accumulate additional heat, leading to raised body temperature. Exposure to heat stress decreases feed intake and thus alters their metabolism and hormonal balance, compromises lactation and reproductive performance and increases disease incidence (WEST, 2003). Traditionally, it has been assumed that heat stress has an indirect effect on bovine production, particularly milk yield, by the reduction in feed intake. However, recent studies have demonstrated that heat stress might have a direct influence on the systemic metabolism, affecting cellular physiology (RHOADS et al., 2009; WHEELOCK et al., 2010). The transition period is critical for a cow’s health and production, even in normal environmental conditions. Nutrient demands during the transition period dramatically change in order to meet the energy (glucose and amino acids) and mineral requirements needed for the gravid uterus in late gestation and for the mammary gland at the beginning of lactation. These demands are maintained by metabolic adaptations in glucose, fatty acid, and mineral metabolism (BELL, 1995). Increased glucose requirements during late pregnancy and the even greater requirements of the mammary gland during lactation demand an adjustment between glucose synthesis and its utilization in the peripheral tissues such as the maternal liver, adipose tissue, skeletal muscles and other tissues (BELL and BAUMAN, 1997). The adaptation of glucose metabolism is supported by increased gluconeogenesis from propionate, amino acids from protein degradation and glycerol released during lipolysis (REYNOLDS et al., 2003). Hepatic gluconeogenesis is increased during late pregnancy and early lactation to supply uterine and mammary demands. Simultaneously, glucose utilisation by adipose tissue and skeletal muscles is reduced (BELL and BAUMAN, 1997). Glucose is the main substrate for lactose synthesis by the mammary gland during lactation (GUINARDFLAMENT et al., 2006). In early lactation, high nutrient demands for lactogenesis usually exceed nutrient intake, resulting in a negative energy balance (NEB) with a marked lipid mobilization from adipose tissue to support milk production (DRACKLEY et al., 2001). Lipid mobilisation during NEB is accompanied by increased non-esterified fatty acid (NEFA) concentration in the blood. NEFA is metabolised in the liver by β-oxidation to acetyl-coenzyme A (acetyl-CoA) that enters the tricarboxylic acid (TCA) cycle to produce energy. Excessive amounts of acetyl-CoA are converted by ketogenesis to acetoacetate, and subsequently to acetone and betahydroxybutyrate (BHB), providing a significant source of energy for transitioning dairy cows (WHEELOCK et al., 2010). Both NEFA and BHB are considered as good markers of excessive NEB and lipomobilisation syndrome (CHAPINAL et al., 2011). A recent paper by MARCO RAMELL et al. (2016) reviewed the potential of proteomic techniques, to search for new biomarkers related to NEB and fatty liver syndrome. Heat stress may further aggravate the adverse effects of NEB, and trigger more serious disorders and systemic inflammation. Thus, proper nutritional management of a periparturient cow is crucial to support metabolic adaptation and to avoid metabolic diseases as much as possible. The aim of this study was to evaluate energy metabolism in heat stressed dairy cows during the periparturient period. Materials and methods Animals and experimental design. Twenty-four Simmental dairy cows were included in the study. The cows were assigned into two groups according to the season: the summer group (n = 12) consisted of cows with calving time during the summer (June, July, and August), and the autumn group (n = 12) consisted of cows that calved during the autumn (October, November and December). The cows were housed on a private dairy farm in Zagreb County, in a partially open free-stall barn with a R. Turk et al.: Energy metabolism in heat stressed dairy cows Vet. arhiv 90 (1), 1-10, 2020 3 ventilation opening on the roof to provide fresh air and to remove excess moisture and heat. Air temperature and relative humidity in the stall were recorded at each blood sampling, using a multi-function climate-measuring instrument (Testo 445, Melrose, MA, USA). From the recorded data, a temperature-humidity index (THI) was calculated according to the equation (NRC, 1971): THI = (1.8 × T + 32) − [(0.55 − 0.0055 × RH) × (1.8 × T − 26.8)]

The influence of heat stress on energy metabolism in Simmental dairy cows during the periparturient period