Abstract
Abstract. 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.
Highlights
In the course of climate change, it is assumed that in regions traditionally characterized by less extreme climate conditions, cows will be faced with temperatures beyond their »comfort zone« (IPCC 2007)
Several indices were developed to describe the impact of Brügemann et al.: Defining and evaluating heat stress thresholds in different dairy cow production systems heat stress on production and functional traits
An interesting finding from the meteorological point of view is that high summer temperatures are compensated by low values for humidities in the maritime region at the coast
Summary
In the course of climate change, it is assumed that in regions traditionally characterized by less extreme climate conditions, cows will be faced with temperatures beyond their »comfort zone« (IPCC 2007). Bohmanova et al (2007) compared several THI formulae for different regions in the USA based on the decline in daily milk yield They identified differences in thresholds for the onset of heat stress for different combinations of regions and indices (Bohmanova et al 2007). Further possible indices like the comprehensive climate index (CCI, Mader et al 2010) consist of temperatures adjusted for solar radiation, wind speed, and relative humidity. Such an index covers a wider range of environmental conditions, but index complexity complicates analyses of variances using random regression methodology. Solar radiation and wind speed are of minor importance for cows kept in insulated barns, implying that there is additional uncertainty, if detailed information for management and housing conditions of farms is missing
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