What is heat stress?
Cattle have a wide tolerance of temperature variations. The comfort zone, or thermoneutral zone, for dairy cattle ranges from -15°C to +25°C. Temperatures either side of this will affect their behaviour, physiology and production (1). Heat wave effects on cattle can have dramatic impacts on animal welfare and performance.
Below the Lower Critical Temperature (-15°C) cattle will eat more dry matter to keep warm, converting more feed into heat rather than milk or muscle growth (1, 2). Above the Upper Critical Temperature (25°C) as much as 20% of the animal’s energy intake will be used on thermoregulation, feed intake drops, and rumination decreases (1). While cattle tolerate cold conditions well, heat stress is generally the more immediate production-limiting factor in UK systems.
Heat stress is not limited to lactating cows. Calves may spend less time feeding during hot weather, while growing beef cattle and replacement heifers can experience reduced daily liveweight gain. Heat stress during the rearing period may also have longer-term impacts on future performance, making early intervention important across all age groups.
Why Heat Stress Matters
Heat stress affects animal performance and welfare and ultimately has economic impacts on the farm.
Reproduction
Conception rates can fall below 35% in heat stressed cows, with higher rectal temperatures and reduced mounting activity contributing to this (3). Research has suggested that cows in winter have about a 3x higher pregnancy success rate during the breeding window compared with those in summer (4).
Milk Yield and Performance
Milk yield and quality can be severely impacted by heat stress with milk fat, protein and non-fat solids all decreasing (3). Dairy cows can reduce milk yield by as much as 12 kg/cow per day compared with cows in the thermoneutral zone (5).
Over a 6-month monitoring period (May–September), one study found heat stress resulted in estimated milk losses of 129–138 litres per cow (6). In the US state of Louisiana, where minimum annual temperatures are as high as 22 °C and maximum relative humidity 92%, milk yield is reduced by more than 2000 kg/cow per year (7).
In beef cattle, heat stress impairs reproductive performance, decreases growth rate, and reduces meat quality (8).
Health and Nutrition
White blood cell counts can increase by as much as 26% in heat stressed cows whilst lameness, ketosis, mastitis and parasite burden also rise due to increased standing, impaired immunity, and more favourable parasitic temperatures (9).
The functioning of the rumen is also negatively impacted in animals under heat stress. This includes lowered rumen motility, an increased concentration of ruminal lactate, decreased pH, altered rumen microbiome, and a higher incidence in metabolic disorders (10).
Behavioural Changes
In hot conditions, cattle reduce their lying time by as much as 28% (11) to increase their surface area. Water intake can increase by up to 2 kg per °C rise (3) and feed intake is reduced by 12% in beef cattle and 40% in dairy cattle (12).
Recognising the Signs of Heat Stress
- Increased respiration rate or laboured breathing
- Open-mouth panting
- Lethargy and reluctance to move
- Reduced feed and water intake
- Standing with heads lowered or grouped closely together
- Elevated body temperature
- Drop in milk production and fertility
- Poor rumination or higher somatic cell counts in dairy cattle
- In severe cases, collapse or death
Practical Strategies to Reduce Heat Stress
Being observant and having animal husbandry skills can help to manage and reduce incidences of heat stress.
Monitoring and Early Intervention
Heat stress is affected by humidity as well as temperature. The temperature-humidity index (THI) can be used to calculate the conditions under which cattle will experience heat stress. Once THI exceeds around 68–72, cattle begin to experience mild heat stress, with production losses increasing as values rise further (14).
In UK conditions, humidity can significantly increase the effective heat load on cattle, meaning animals may experience heat stress even at relatively moderate temperatures of 19–22°C when humidity is high.
Monitoring livestock to catch heat stress early can involve simple observations such as rectal or infra-red temperature readings and panting scores to high-tech monitoring devices such as ear tags, image analysis and accelerometer-based sensors (15).
Diet
When feed intake drops due to heat stress, nutrient concentration should increase. Higher quality forages and concentrates can help maintain energy requirements (1). Feeding fat (not exceeding 5-7% of the diet) or ionophores during times of heat stress can increase milk yield (3). Furthermore, changing the feeding regime, for example, providing most of the feed overnight when temperatures are cooler, can improve feed intakes.
Environmental Management
The combination of fans and sprinklers in collecting yards has been shown to lower cow body temperatures by 1.7°C, leading to improved milk yields compared with unmanaged cows (1). Air movement should be around 1.5–2 m/s to improve convective cooling and evaporation from the cow (16).
Housing design plays a critical role in mitigating heat stress in UK systems. Many dairy sheds rely heavily on natural ventilation, and while this is often sufficient for air exchange, it may not provide adequate air speed at cow level during hot weather. Reducing stocking density, opening side sheets, and ensuring ridge ventilation is unobstructed can all improve airflow and support natural ventilation via the stack effect.
Reduce handling times, walking distance, and time spent in holding pens by as much as possible during hot weather (16). Finally, access to fresh, clean water at all times should not be overlooked. Competition at water troughs can be a limiting factor so providing enough troughs and ensuring adequate flow rates is essential to maintaining both welfare and productivity during warm periods.
Conclusion
With rising temperatures and heatwaves becoming more common and prolonged, UK farmers need to adapt to combat the associated impacts on the cattle industry. Without drastic intervention, it is predicted that the cost of heat stress production losses in the UK livestock industry would be £5.8 million by 2080 and £34 million in mortality losses (17).
Ultimately, simple interventions such as cooling systems, improved ventilation, and smart feeding strategies can significantly reduce losses and improve both welfare and productivity in the cattle industry during hot weather.
Sources
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Morignat, E., Gay, E., Vinard, J.L., Calavas, D. and Hénaux, V., 2015. Quantifying the influence of ambient temperature on dairy and beef cattle mortality in France from a time-series analysis. Environmental research, 140, pp.524-534.
Becker, C.A., Collier, R.J. and Stone, A.E., 2020. Invited review: Physiological and behavioral effects of heat stress in dairy cows. Journal of dairy science, 103(8), pp.6751-6770.
De Vries, A. and Risco, C.A., 2005. Trends and seasonality of reproductive performance in Florida and Georgia dairy herds from 1976 to 2002. Journal of Dairy Science, 88(9), pp.3155-3165.
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Das, R., Sailo, L., Verma, N., Bharti, P., Saikia, J. and Kumar, R., 2016. Impact of heat stress on health and performance of dairy animals: A review. Veterinary world, 9(3), p.260.
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