Efeitos do estresse por calor no desempenho e contagem de células somáticas de vacas leiteiras

Abstract

Heat stress in dairy cows is one of the main factors affecting the welfare and productive performance of these animals, especially in tropical and subtropical regions. The objective was to evaluate the effects of thermal stress on physiological, immunological (SCC), productive, and qualitative variables in dairy cows of different breeds and production systems. The first chapter aimed to synthesize, through a narrative review, the main factors that influence heat stress in dairy cows. The second chapter focused on conducting a systematic review with meta- analysis. To formulate the guiding research question, the PICOS strategy was used, where the population (P): dairy cows; intervention (I): heat stress; comparison (C): thermal comfort; and expected outcomes (O): milk yield and composition, and somatic cell count. Only well- designed experimental studies were accepted as the study type (S). The databases SCOPUS (Elsevier), CAB Direct, Science Direct (Elsevier), and Wiley Online Library were used for the search. The review initially identified 1,835 articles, of which 24 were considered eligible for data extraction due to their methodological robustness. Articles were included when they addressed the influence of the Temperature–Humidity Index (THI) on thermoregulation, milk yield and composition, and when they provided different strategies for mitigating heat stress. The results confirmed that an increase in THI above 68 triggers heat-dissipation mechanisms, leading to negative impacts mainly on milk production. A reduction in milk protein and fat contents was also observed, along with changes in somatic cell count (SCC). It is concluded that heat stress has negative effects on the metabolism and immunity of dairy cows, making the adoption of mitigation strategies—such as natural or artificial shading and cooling—essential to ensure cow welfare and productivity. Keywords: dairy cattles; heat stress; milk production; systematic review; thermal comfort.