Heat Stress in Dairy Cows: A Growing Concern

Ishaya Gadzama

Research Scientist

13 min read
15/01/2025
Heat Stress in Dairy Cows: A Growing Concern

Introduction

Heat stress is a primary concern for dairy farmers worldwide, and it's becoming more of a problem, especially with rising global temperatures. It's not just about the cows feeling uncomfortable; it also leads to big losses in milk production and affects their overall health. Heat stress is probably one of the most investigated problems in the dairy sector, and it is attracting more and more attention from researchers and farmers. Heat stress often leads to decreased milk production and protein concentration (Chen et al., 2024). Understanding and managing heat stress is essential for keeping dairy herds healthy and productive. Various remote sensing technologies, such as ear canal sensors, rumen boluses, rectal and vaginal probes, infrared thermography (IRT), and implantable microchips, can be used to evaluate the degree of heat stress experienced by grazing animals (Hoffmann et al., 2013; Koltes et al., 2018; Liu et al., 2019; Chung et al., 2020; Wijffels et al., 2021). The Temperature Humidity Index (THI) is a commonly used measure to assess the level of heat stress in animals (Dash et al., 2016; Liu et al., 2019). 

What is Heat Stress?

Dairy cows build up heat from their metabolism and the environment. Heat stress occurs when cows are unable to effectively dissipate (get rid) of that heat (Polsky and von Keyserlingk, 2017), leading to a rise in their body temperature, and it's a big challenge for dairy farmers, especially in hot and humid climates (Habimana et al., 2023). This happens when the environmental conditions, particularly high temperature and humidity, exceed the cow's ability to regulate its internal temperature. Different formulas may be used to measure heat stress in various regions and under different climatic conditions. 

Temperature Humidity Index

The temperature-humidity index (THI) is a common way to measure heat stress in cattle (Dash et al., 2016; Liu et al., 2019). This index combines both the air temperature and the relative humidity of the environment into one single number (Talukder et al., 2024). When it's hot and humid, the THI goes above a certain level, and cows struggle to maintain their normal body temperature, indicating higher heat stress, which can lead to several problems (Dash et al., 2016; Polsky and von Keyserlingk, 2017; Liu et al., 2019). The THI can be calculated using various formulas.

Calculation of Temperature Humidity Index

The most common formula used to measure heat stress in animals is based on the National Research Council (NRC): THI = (1.8 × T + 32) - (0.55 - 0.0055 × RH) × (1.8 × T + 32 - 58), where T is the temperature in degrees Celsius, and RH is relative humidity. Some studies used a slightly different version of this equation: THI = (1.8×T+32)–(0.55–0.0055×RH)×[1.8×T–26], where T is the temperature in degrees Fahrenheit (Chen et al., 2024). This particular version is also attributed to the NRC (1971). Over the years, other formulas have been developed. For example, Yousef (1985) proposed a formula using dry and wet bulb temperatures: THI = 0.72 (Tdb + Twb) + 40.6, where Tdb is the dry bulb temperature, and Twb is the wet bulb temperature in °C (Yousef, 1985). In addition, the THI can be calculated using either daily mean or daily maximum temperatures and humidity. However, daily mean THI is generally considered to be a good indicator of heat stress (North et al., 2023). Daily mean THI also indirectly incorporates night-time cooling, or whether the cattle experience relief from heat stress at night (North et al., 2023). It is important to know that the THI is not a perfect measure, as it doesn't take into account factors like solar radiation and wind speed that can also affect how a cow feels (Leandro et al., 2024). It's often used, though, because it's relatively easy to calculate using temperature and humidity readings from weather stations (Chen et al., 2024; Talukder et al., 2024). The best practice is combining environmental measures like the THI with animal-based measures such as body temperature, and behavior provides a more comprehensive understanding of heat stress in dairy cows. This approach can help farmers make more informed decisions to ensure the well-being and productivity of their herds (Liu et al., 2019).

Threshold of Heat Stress 

A THI above 72 is generally considered the threshold where heat stress begins to affect cows (Dash et al., 2016), meaning that when the THI goes above 72, it's likely that the cows will start to feel the effects of the heat (Chen et al., 2024). However, some studies use a lower threshold of 68 (Leandro et al., 2024). 

Physiological Responses to Heat Stress

When dairy cows experience heat stress, their bodies initiate a cascade of physiological changes to try and maintain a stable internal temperature, also known as homeothermy (Kadzere et al., 2002; Galán et al., 2018). This is because dairy cows, like other homeotherms, function best when their core body temperature is within a narrow range (Kadzere et al., 2002). Increased body temperature is one of the most immediate responses to heat stress (Liu et al., 2019). When the ambient temperature rises above a cow's comfort zone, she struggles to dissipate heat effectively (Liu et al., 2019). This increase in body temperature indicates that the cow is experiencing heat stress (Liu et al., 2019).

Cows also reduce their metabolic activity during heat stress, including releasing thyroid hormones related to metabolic heat production (Becker et al., 2020; Tao et al., 2020; Leandro et al., 2024). This helps minimize internal heat generation but comes at the expense of milk production and feed intake (Becker et al., 2020). Changes in milk production and components like protein and fat concentrations can help understand how heat affects dairy cows (Pryce et al., 2022; Chen et al., 2024).

Direct Physiological Measurements

Researchers directly measure how cows respond to heat by looking at their body temperature (Liu et al., 2019). This can include rectal temperature, which is a core body temperature measure. However, it's invasive for the cow, and more recently, vaginal temperature has become a common way to monitor heat stress in dairy cattle (Bohmanova et al., 2007; Galán et al., 2018; Wijffels et al., 2021). Heat stress affects oxygen metabolism and transport in dairy cows (Almoosavi et al., 2021). This means that the cows have a reduced ability to deliver oxygen to the body tissues, which can lead to decreased productivity (Sharma et al., 2011). To cool down, cows will increase their respiration rate by panting (Rhoads et al., 2009; Galán et al., 2018). Panting indicates that cows struggle with heat (Herbut et al., 2019). While it helps release heat through the evaporation of moisture from the respiratory tract, it can also disrupt the balance of carbonic acid and bicarbonate necessary to maintain blood pH, leading to respiratory alkalosis (Dalcin et al., 2016; Galán et al., 2018).

Scientists also keep an eye on a cow's respiratory rate (how fast she’s breathing) and whether she's panting (Galán et al., 2018; Liu et al., 2019; Pryce et al., 2022). The body also redistributes blood flow from the gastrointestinal tract to the skin surface in an effort to cool down (Galán et al., 2018). Surface body temperature can be measured using infrared cameras, a non-invasive way to see how hot a cow's body surface is (Koltes et al., 2018). Specific body areas like the eyes, forehead, flank, and udder are focused on getting a reading of body temperature (Kadzere et al., 2002; Sejian et al., 2022). Some studies are exploring the use of milk temperature as an indicator of heat stress, as it's easier to monitor than other temperature measures (Liu et al., 2019).

Behavioral Observations

Changes in behavior can tell farmers a lot about heat stress in their cows. For example, cows tend to eat less dry feed (DMI) when they're hot (Chen et al., 2024). This is because digesting food generates internal heat, so eating less helps reduce the heat load (Becker et al., 2020). However, a reduction in feed intake also means that the cow gets less nutrients and energy, which affects milk production (Chen et al., 2024). When cows are heat-stressed, they also spend less time ruminating (chewing their cud). Rumination is important for digestion, and a decrease in this activity can indicate a drop in well-being (Talukder et al., 2024). Therefore, researchers monitor feed intake and rumination as indicators of heat stress.

Researchers are studying the use of activity monitors and other sensor technologies to monitor heat stress in cows continuously (Koltes et al., 2018). It is important to note that different cows respond to heat differently (Pryce et al., 2022). Factors like breed, stage of lactation, and milk yield can all affect how a cow copes with heat (Galán et al., 2018; Chen et al., 2024). The duration of heat stress also matters, with some studies looking at short-term stress in climate chambers and others examining longer periods in real-world conditions (Galán et al., 2018).

Environmental Factors

Studies on heat stress have been conducted in various systems, including free-stall barns, tie-stall barns, and pasture-based systems, indicating that the specific environment is also important (Talukder et al., 2024; Leandro et al., 2024). Even within a small area, different locations can have different temperatures and humidity levels, depending on factors like wind and sun exposure (Leandro et al., 2024).

Impact of Heat Stress on Milk Production

Heat stress can lead to a reduction in milk yield and changes in milk composition (Pryce et al., 2022; Chen et al., 2024). Producing milk requires energy and nutrients, so when cows are stressed by heat, they prioritize survival over milk production (Rius, 2019). This results in decreased milk yield and often a drop in milk protein concentration (Chen et al., 2024; Leandro et al., 2024). Studies have shown that milking frequency increases as THI rises until a THI of 90 is reached, after which it starts to decline, indicating a response to heat stress (Talukder et al., 2024).

Adaptation Mechanisms

Dairy cows have developed several coping mechanisms to deal with heat stress, including acclimation, acclimatization, and adaptation (Beatty et al., 2006; Bernabucci et al., 2010). 

Acclimation refers to the physiological changes that occur in response to a single stressor, such as high temperature (Beatty et al., 2006; Bernabucci et al., 2010).

Acclimatization involves adjustments to a combination of environmental stressors, like temperature, humidity, wind, and solar radiation (Beatty et al., 2006; Bernabucci et al., 2010).

Adaptation is a long-term genetic change to cope with environmental pressures and stressors (Beatty et al., 2006; Bernabucci et al., 2010). Cows will also modify their behavior to mitigate heat stress. They may spend more time standing, which increases their surface area for heat loss (Galán et al., 2018). They may also reduce their physical activity to minimize internal heat production (Galán et al., 2018).

Cows often seek shade to reduce exposure to direct solar radiation and increase their water intake, sometimes drinking more than usual to compensate for increased water loss due to sweating and panting (Galán et al., 2018; Golher et al., 2021). They also change their feeding behavior by eating less and reducing the time they spend eating (Galán et al., 2018).

Genetic selection can affect a cow's ability to tolerate heat (Santana et al., 2017; Garner et al., 2017). Some breeds are naturally more heat-tolerant than others, and heat tolerance within breeds can vary (Garner et al., 2017).

At the cellular level, heat stress can induce a stress response that involves the production of heat shock proteins which help protect cells from damage and help them survive during periods of heat stress (Kim et al., 2022).

Conclusion

Dairy cows respond to heat stress with complex physiological changes and adaptation mechanisms. These include changes in respiration, body temperature, feed intake, milk production, and behavior. These responses help cows maintain thermal balance and survive stressful conditions. Researchers use various tools to measure heat stress in dairy cows. These tools include indices such as the Temperature-Humidity Index (THI), direct physiological measurements, behavioral observations, and other indicators. Understanding these mechanisms is critical for developing strategies to mitigate the negative effects of heat stress on dairy cow health and productivity.

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