Reduce losses associated with heat stress

Summer in the UK has the potential to produce some of the most stressful environmental conditions of the year for both housed and grazing cows. The combination of heat and humidity limits the ability of the cow to successfully thermo-regulate. Heat stress limits the ability of the cow to maintain milk production, body condition, fertility and rumen health.

Clinical Signs

Clinical signs of heat stress include panting, reduced lying times and milk fat depression among others. If these clinical signs are apparent, the herd is already losing vital margin on milk sales. Dairy herds are also at risk of long term health and fertility problems caused by heat stress.

Effects of heat stress


Figure 1 shows the pregnancy risk of two herds, one fully housed and the other grazing. In both examples it is clear that the summer months of May, June, July, August and September are on average the poorest months in terms of pregnancy risk. There is variation in the spread of data, but it is clear that both herds are being affected by heat stress.

It is also evident that it is both submission rate and conception rate that decline in these months. Heat stress reduces bulling activity making it more difficult to observe cows bulling and can induce periods of anoestrus. Following periods of heat stress an increased rate of twinning is often experienced.

Behavioural changes in cows affected by heat stress are all associated with attempts to increase the loss of heat from their body. Panting, salivating and bunching up in shaded or

Figure 1. insemination and pregnancy rate in two herds

cooler areas is common. Cows are able to dissipate more heat when stood up, simply because more of their body’s surface area is in contact with the air.


Reduced bulling activity is associated with increased lethargy during periods of heat stress and is compounded by negative energy status. Rumination produces a large amount of heat. Heat stressed cows will sort feed more aggressively, avoiding fibre, therefore reducing rumination rates and heat production. The consequence of this can often be reduced intake, supressed milk fats and ruminal acidosis. Evidence of this can be seen in many herds.

Figure 3 shows the correlation between temperature and fat/protein ratio observed on a farm suffering heat stress. Poor fat to protein ratios are observed in both indoor and grazed herds suggesting that environmental and not nutritional factors such as the introduction of grazed grass are responsible.

fat protein chart
Figure 2 Fat: Protein ratio and air temperature in a housed herd from May to November

During periods of heat stress udder health may also be compromised due to its negative effect on immunity. Increased standing times and reduced lying times are common in heat stress conditions which can lead to an increase in lameness. Growth rings on feet is evidence of historic heat stress, showing times when foot horn development was compromised.

Heat stress leads to a reduction in dry matter intake, along with increased risk of sub-acute ruminal acidosis, both of which can lead to excess body condition loss. This can predispose to substandard reproductive performance. Failure to get sufficient numbers of cows pregnant will reduce milk sales and profitability.

signs of heat stress
Figure 3: Signs of heat stress (Lallemand)

Figure 3 shows the full range of symptoms and hidden losses associated with heat stress. The losses are all consequences of increased demand on metabolism as cows struggle to maintain homeostasis.

There is overwhelming evidence of heat stress on UK dairy farms. The point at which cows are exposed to such conditions varies depending on the metabolic load of the cow, the environment in which she is managed and the Temperature Humidity Index (THI). The THI is a factor of air temperature and humidity.

High humidity reduces the ability of the cow to dissipate the heat she produces.  Figure 4 shows the levels of temperature and humidity that predispose to heat stress.

levels of risk
Figure 4. THI values and the level of risk associated with specific environmental temperatures and humidity

As can be seen from Figure 4 a THI value over 68 puts the cow at risk of heat stress. Under UK conditions, humidity has a significant part to play in exposing cows to heat stress. It is important to remember that these measurements need to be taken in the cow’s environment, so if you see steam rising from cows’ backs in a holding yard, you can be sure the humidity and temperature are high.

Studies carried out in UK collecting yards in 2015 have shown regular humidity levels of 90%, which would need only a temperature of 22˚C to put every cow in the herd under heat stress conditions. In these studies, milk loss was calculated at 2.5L/cow/day.

Other peer reviewed studies by Shuller et al (2016) reported cows exposed to one incident of heat stress in over a 21 day period were 63% less likely to conceive. Karimi et al (2015) concluded that dry cows exposed to heat stress produced 10% less milk in early lactation and suffered from reduced dry matter intakes. Reducing exposure to heat stress has been proven to improve herd performance.

When applying this theory to UK dairy systems, consider risk areas for the herd. How is air going to get in to buildings and across cows? There will be year round benefits of improving airflow in sheds, as improving air quality improves cow health and milk production 365 days a year.

Think about areas and times of high heat stress, with the most obvious area being the collecting yard. Cows may not be under heat stress conditions all day, but as they enter the collecting yard, they tip into heat stress. In these situations, it takes a period three times longer for the cows to come out of heat stress as the period of exposure.

This means a cow under heat stress for 2 hours a day, suffers the consequences for a quarter of the day. Simple things like ample supply of water, reducing time cows spend in collecting yards and basic fan setups can reduce the risk exposed to your herd. Use sprinklers with caution as wetting cows in humid conditions can be counter-productive.


In times of tight on farm margins, consideration of heat stress can be the difference between profit and loss. Slight building alterations and a small investment in fans could be the difference between achieving and not achieving fertility targets.

As a rule of thumb a £5000 investment in a 200 cow herd would be paid for in one summer if the effects of heat stress on yield and fertility were mitigated. Most cows will be exposed to heat stress sometime or other, if it is your most limiting factor, the return from investment will soon pay off.

Mark Price

Dairy Specialist

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