Heat Stress for Cattle - Keeping Cows Cool

Dairy cows have a broad thermoneutral zone (the range of temperatures where they do not need to use energy to regulate their temperature) from -15C to 25C. When cows reach their upper critical temperature, they have two methods to prevent their core body temperatures from rising: increasing heat dispersion and limiting heat production.
They can disperse heat by physical changes such as increased blood flow to their skin, panting and salivating. They can limit heat production by reducing their physical activity and decreasing their dry matter intake. The majority of heat produced by a cow is through rumen fermentation and rumination so reducing her dry matter intake reduces both of these.
However, it is the interaction between temperature and humidity which leads to signs of heat stress in dairy cattle. Temperature Humidity Index (THI) gives us a way to describe this interaction, and it is accepted that THI > 70 leads to signs of heat stress in cows.

What is the interaction between temperature and humidity that leads to heat stress?

 Humidity has a big impact on cows’ ability to dissipate heat, so as humidity increases, the temperature at which a cow starts to display signs of heat stress reduces.
In the UK, relative humidity is frequently over 80% in summer months - and in buildings without adequate ventilation this can reach 100% - so cows can start to exhibit signs of heat stress at temperatures below 20C.

What are the impacts of heat stress on the milking herd, dry cows?

Milk Production
High yielding cows are at greater risk of heat stress than lower yielding or dry cows, due to the increased feed requirements leading to greater heat production from the rumen. A first reaction to heat stress is to reduce dry matter intake, which in turn leads to reduced milk yield. The cow is also utilising more metabolic energy to try to maintain its temperature, again diverting energy away from milk production.

Fertility
Heat stress has a negative impact on pregnancy rates with a reduction in both conception rate and submission rate seen during summer months. Duration and intensity of bulling behaviour is reduced in heat stressed cows which may in part be because of reduced feed intake but also due to lethargy being part of the cows behavioural coping mechanism for the excess heat.

Lameness
Behavioural changes associated with heat stress are not limited to reduction in oestrus activity. Heat stressed cows have reduced lying time as they attempt to increase heat dissipation from their bodies with some studies reporting 30% reduction in lying times. Increased standing times are associated with a greater risk of lameness, particularly sole ulcers.

Heat stress and Dry Cows
Focus is often placed on the milking herd but there are impacts on dry cows too. Dry cows that were heat stressed for all, or part of the dry period, went on to give less milk than cows that were cooled through their dry periods.
Studies from the USA have shown that calves born to heat stressed cows have lower birth weight, lower weaning weight and poorer colostrum absorption. The impact of heat stress appears to continue affecting calves born to heat-stressed dams, as they tend to have lower weights from birth through to bulling. Additionally, these calves are more likely to leave the herd before their first lactation, and those that do join the milking herd typically produce lower yields compared to calves born to non-heat stressed dams. The mechanism behind this is not fully understood but is thought to be due to reduced dry matter intakes, effects on the placenta and changes to genetic programming of the fetuses while their dams are subjected to heat stress.

Advice on managing/mitigating heat stress within the milking herd
Shade – providing cows places to get out of direct sunlight can reduce the rate at which they start to show signs of heat stress. Sky lights in buildings can produce ‘hot spots’ so particular cubicles can be much more uncomfortable than others, leading to some producers ‘blacking out’ existing sky lights to provide a more uniform environment.
Ventilation – this describes moving fresh air into the building and can be mechanical or natural. Moving fresh air into the building can help remove moisture and reduce the humidity in the building allowing cows to tolerate higher relative temperatures.
Air Movement – increasing the speed of the air moving over the cows can help them dissipate heat more effectively. Using fans, angled to ensure they are blowing onto the cows at air speeds of around 10mph, can be effective in cooling.
Cooling with water-misters and sprinkler systems can also be effective ways to help cows dissipate heat however they do significantly increase the humidity - which may have unintended consequences, such as improving the environment for mastitis pathogens to multiply. They may have a role in areas where cows congregate, such as the collecting yard.

Drinking water – you should provide 10cm water trough space per cow, with sufficient water flow to meet the highest traffic demands – cows will drink 50% of their water intake immediately post milking.

What is the role of fans in managing heat stress and for farmers looking to install fans in cattle sheds - what should they be looking for / looking to achieve?

 
Fans can be useful in both cattle buildings and areas where cows are grouped such as the collecting yard. Fans can be fitted with automated switches that monitor temperature and humidity, so the fans come on at the right time and at the appropriate speed for the conditions.
Fan systems can be designed to improve ventilation, which will aid reduction in humidity by introducing fresh air and removing moisture. They can also be designed to increase air movement at the cow level to aid heat dissipation. The number and position of fans should be designed to your specific shed and should consider natural ventilation and prevailing wind conditions.
Collecting yards can become a significant bottleneck for heat stress, as the close proximity of the cattle makes it harder for them to release heat. Therefore, this area could be a target for initial investments.