A systematic review of non-productivity-related animal-based indicators of heat stress resilience in dairy cattle

Abstract

Introduction: Projected temperature rise in the upcoming years due to climate change has increased interest in studying the effects of heat stress in dairy cows. Environmental indices are commonly used for detecting heat stress, but have been used mainly in studies focused on the productivity-related effects of heat stress. The welfare approach involves identifying physiological and behavioural measurements so as to start heat stress mitigation protocols before the appearance of impending severe health or production issues. Therefore, there is growing interest in studying the effects of heat stress on welfare. This systematic review seeks to summarise the animal-based responses to heat stress (physiological and behavioural, excluding productivity) that have been used in scientific literature.

Methods: Using systematic review guidelines set by PRISMA, research articles were identified, screened and summarised based on inclusion criteria for physiology and behaviour, excluding productivity, for animal-based resilience indicators. 129 published articles were reviewed to determine which animal-based indicators for heat stress were most frequently used in dairy cows.

Results: The articles considered report at least 212 different animal-based indicators that can be aggregated into body temperature, feeding, physiological response, resting, drinking, grazing and pasture-related behaviour, reactions to heat management and others. The most common physiological animal-based indicators are rectal temperature, respiration rate and dry matter intake, while the most common behavioural indicators are time spent lying, standing and feeding.

Conclusion: Although body temperature and respiration rate are the animal-based indicators most frequently used to assess heat stress in dairy cattle, when choosing an animal-based indicator for detecting heat stress using scientific literature to establish thresholds, characteristics that influence the scale of the response and the definition of heat stress must be taken into account, e.g. breed, lactation stage, milk yield, system type, climate region, bedding type, diet and cooling management strategies.

Fig 1. PRISMA 2009 flow diagram.

Fig 2. Animal-based indicators (ABIs) of heat stress of dairy cows.

Circle packing plot representing showing how many studies use the most widely used ABIs listed in Table 1 and S3 Table. The diameter of the circles is proportional to the number of occurrences of each ABI included in each aggregation level (bold). The darker the circle, the higher the level of aggregation.

Fig 3. Classification of yields when DIM are reported.

Source: own work from the milk yields and days in milk reported in the studies. From the red line downwards the yields are <22 kg/d using standardised lactations of 305 days. From the green line upwards the yields are >34 kg/d.