Defining and evaluating heat stress thresholds in different dairy cow production systems

The aim of this study was to assess the impact of heat stress in dairy cows on test-day records for production traits and somatic cell score (SCS) in the state of Lower Saxony, Germany. Three different production systems were defined: A production system characterized by intensive crop production (=indoor housing), a pasture based production system, and a maritime region. Heat stress was assessed by two temperature-humidity indices (THI) modelled as random regression coefficients in an analysis of variance: One (THIBo) was defined as an average of hourly THI, calculated from hourly recorded temperatures and humidities, the other (THIRa) was based on daily maximal temperature and daily minimal humidity. In all production systems, THIBo=60 and THIRa=70 were identified as general thresholds denoting a substantial decline in test-day milk yield. For daily fat and protein percentage, no universally valid thresholds were identified. In contrast for SCS, especially in the maritime region, heat stress as well as cold stress thresholds were found. Regression analysis was used to study the change in test day milk yield in response to THI of those THI ranges with an obvious decline in milk yield. Regression coefficients were -0.08 kg/THIBo and -0.16 kg/THIRa for the crop production system, -0.17 kg/THIBo and -0.23 kg/THIRa for the pasture based system, and -0.26 kg/THIBo and -0.47kg/THIRa for the maritime region. Based on statistical information criteria, identified thresholds for THIBo should be given preference over THIRa when applying genetic studies on heat stress in German Holstein cows.