###### Abstract  
In this study, we performed a systematic review to assess the effect of thermal stress on the behavior of lactating cows housed in compost barns. Peer-reviewed articles written in English that evaluated the effect of heat stress on the behavior of lactating cows raised in compost barns were used in this systematic review. The resulting articles (38) underwent a four-step PRISMA appraisal process and resulted in six articles that fitted our inclusion criteria. Our review highlighted that the temperature and humidity index prevailed among the studies to evaluate thermal comfort. Regarding the behavior, heat stress promoted decreases in feed events and the time that cows spent lying down. In contrast, there was an increase in events of visiting the water trough, the number of steps, agonistic behavior, and dyspnea. In conclusion, heat stress affected the behavior of lactating cows raised in compost barns; however, some care is needed in extrapolating our findings since this is a recent research area and further studies are needed. confined system; cattle; temperature-humidity-index; applied ethology; thermal comfort; zero-grazing system +
Footnote †: journal: Medical  
related to heat stress result in annual economic losses of approximately 2.3 billion USD for the American dairy industry [16]. Animal thermal comfort concerns have expanded in the last 25 years among temperate and tropical climate countries [6; 17], as thermal heat load remains a significant problem in production systems, regardless of geographical region. To mitigate the effect of heat stress on dairy cows, confinement systems have emerged as an alternative to pasture-based systems. Among the confinement systems, the compost barn system is a relatively new alternative for dairy farmers [18]. The compost barn system (Figure 1) consists of housing animals in a barn with a resting area covered with an organic compost-bed [19] without stalls or partitions [20]. The main positive points of the use of this system are the improvement in the comfort and welfare of the dairy cows, improvement in milk quality, reduction in hoof problems, increase in reproductive and productive efficiency, low initial construction cost, and better manure management [21]. On the other hand, the compost barn system presents some challenges, such as thermal conditioning and bed management [22]. As the main difficulties of the system highlight seasonal offers of bed materials (especially in the winter), the high cost of bed materials, the use of implements for the revolving of the bed, the use of fans for heat abatement, and the increased use of electricity [21]. One approach for thermal environment control in compost barns consists of the thermal conditioning of the facility, which can be natural or artificial. However, it is important to highlight that the compost barn has two structure forms, (1) a system with open sides and (2) s system with closed sides. Therefore, the inside cooling efficiency is directly influenced by the structure of the barn [24]. Thus, evaluating the effects of thermal conditions on dairy cattle facilities is necessary [25]. This can be explained because the housing structure has the potential to reduce or increase indoor climatic conditions [26; 27] and consequently, a better or worse environment induces behavioral changes in dairy cows. In summary, the behavior of cows is expressed in values that reflect the time of their activities. In comfort conditions, cows lie down for an average of 10-12 h/day, and a reduction in this behavior can be related to welfare problems [28], lameness [29] and metritis [30]. In contrast, cows in heat stress decrease dry matter intake (approximately 50%; [PERSON] et al. [31]), are prone to reduce resting time lying down [32] and consequently increase the probability of standing up [33]. This change in posture may be related to aiding body heat abatement [34]. The change in behavior is the cow's first reaction to the challenging environment (e.g., heat stress).