Heat stress in dairy cattle: What is it and what are the consequences?

As most of us have experienced in recent years, the climate is causing warmer summers with higher temperatures. While some might love it, it is often a difficult period for you as a farmer. It is a hectic period, with the busy time on the land, along with the cows that are also bothered by the hot days. Together, this calls for good management during this period. Especially for the cows, it is important that occurring issues need to be addressed in the right way, because it might have a major impact on the further lactation. At AHV, we will be more than willing to help you and your herd get through the warm period.

Energy distribution in dairy cows

In cows, available energy is distributed according to priority processes. In a healthy cow, this energy goes first to maintenance, then to product, next to growth, and finally to reproduction, immunity, and thermoregulation. If their energy intake is limited or more energy is allocated to health issues, there will be consequences. Take for example a cow that has had a heavy calving and therefore will not clean up properly. In that case, a major part of the energy goes to her recovery and uterine cleaning. As a result, less energy will be available to start up milk production. That is why it is often reflected in a poorer start of lactation.

However, you can also expect reduced performance in terms of growth, fertility and immunity. In warm weather conditions, the likelihood of health issues is also higher. This is mostly due to deteriorated energy management and occurrence of heat stress. But when do we actually speak of heat stress in dairy cattle?

What is heat stress?

Every animal has a thermoneutral zone, the interval of ambient temperature within which the animal does not need to spend extra energy to keep its body temperature constant. For the cow, the thermoneutral zone lies between -10 and 22 degrees Celsius (°C) (Kempenaar & Van Dooren, 2003).

At temperatures within this thermoneutral zone, the cow thrives best and there are no direct effects on its energy balance. Outside of this thermoneutral zone, cows consume extra energy to actively warm up or cool down. Which is at the expense of the energy available for milk production, growth, fertility and immunity.

Due to their high heat production and feed intake, lactating animals are much more susceptible to heat stress than young stock and beef cattle for example. Other animals that are considered risk-animals in case of high temperatures are fresh cows, high-producing cows and dry cows. Mainly because these animals are often not in an optimal energy balance already.

Indicators of heat stress

Heat stress is generally defined as a situation in which there is an imbalance between the cow’s heat production and its ability to release this heat to the environment. Heat stress occurs once the upper critical temperature of the thermoneutral zone is exceeded. To determine whether a particular climate can cause heat stress in cows, heat stress indicators such as the Temperature Humidity Index (THI) are used. The THI uses the two most important parameters, namely temperature and relative humidity, to measure the effect of climate on animal response (Aggarwal & Upadhyay, 2013).

Figure 1 shows the Temperature Humidity Index (THI) used for dairy cattle. In this figure, green indicates no heat stress, yellow indicates mild heat stress, orange means heat stress, dark orange severe heat stress, and red lethal heat stress.

Figure 1: Temperature Humidity Index (THI) based on the formula as used by Van Laer et al. (2015) with threshold of Zom (2016). (Relatieve luchtvochtigheid = Relative humidity).

Dairy cows experience heat stress at a THI of 68 and above (see figure 1), while for beef cattle is it is at a THI of 78 and above. The THI does not take into account solar radiation, cooling by wind or duration of heat stress.

The impact of warm temperatures on your livestock do not always show immediately. It can take over a week before you experience the effects of the warm period. It is therefore important to act proactively before and during a warm period. This starts with keeping an eye on (upcoming) weather and the animals at risk. In this way, it is possible to anticipate the cow’s energy balance in time. This gives your cows a better chance of getting through the warm period and staying healthy.

At AHV, we are committed to keeping cows healthy and want to tackle health challenges as early as possible. That is why we have listed some key indicators that can be of help in monitoring your cow’s overall health status (in periods of heat stress). However, these indicators will always remain dependent on temperatures and you cow’s history, but it will give you a good basis.

1.      Feeding time

Feeding time is defined as the time spent on prechewing, chewing, and swallowing feed. Feeding time is strongly influenced by factors such as feed management, dry matter intake (DMI), and ration composition. In addition, foraging time can differ between animals (Florit et al., 2023). The average feeding time of a cow is between 6 and 9 hours per day.

2.      Rumination time

On average, an adult dairy cow spends about 7 to 9 hours a day ruminating. Per kilogram of roughage, this is between 25 and 80 minutes (Paudyal, 2021; Byskov et al., 2015).

3.      Number of ruminations

The number of ruminations per rumination chunk gives an indication of the structure of the ration and the rumen health of the cow; a minimum of 60 ruminations is the target value.

Consequences of heat stress in dairy cattle

Cows can show reduced feed intake of 10% or more as early as 20°C. As a result, less energy is available for milk production, for example. Consequently, you see that milk production can start to drop by 15 to even 40%.

Hence, at temperatures above 18°C, it is important to be alert to any consequences of heat stress. This is because of the major impact on the animal’s general health. The following aspects are most affected by heat stress:

• Production;
• Immunity;
• Ruminant function;
• Claw problems;
• Udder health;
• Fertility (Timmerman et al., 2018).

Extra attention for dry cows during warm periods

Whereas the warm period is already a challenge for any cow, it is certainly the case for dry cows. A relatively short period of high temperatures in late gestation can greatly affect not only the health of the cow, but also that of the calf. It is crucial to avoid negative effects on the calf. When a pregnant cow has not been affected by the warm weather, the birth weight of the calf is on average three kilos heavier. In addition, these calves have a better immunity. This is because the colostrum contains more antibodies and can be absorbed better. For this reason, these calves appear to absorb 43% more antibodies (IgGs) from the same colostrum than calves from cows that did experience heat stress (Monteiro et al., 2016). In addition, we see that the failure rate until the end of the first lactation is more than twice as high and milk production is 16% lower.

Tackling heat stress proactively among your herd

When a cow suffers from heat stress, a drop in feed intake can be observed, and that while she is in need of extra energy and protein. Especially the energy and protein intake are important to ensure that she continues to feed well and can use the energy gained from the ration to maintain the various body processes. Since cows feed less during periods of heat stress, it is crucial to provide extra energy. This should be done before it is too late, to avoid the energy dip before it occurs.

Continuously encouraging feeding

At AHV, we believe it is best to proactively support animals at different times in her cycle. We therefore recommend using our Booster products when heat stress is expected. Provide a Booster Tablet to high risk animals one day prior to a warm period to make energy available in advance and stimulate feeding behaviour. The AHV Booster Tablet optimises the energy balance by activating the cow’s metabolism (liver and rumen). It will help the liver to prevent fat breakdown. Especially for high-risk animals such as fresh, high-yielding and dry cows, this support is extra important.

For all cows, we recommend proactive support with AHV Booster Powder. To ensure that your cows receive extra energy through the ration, we recommend adding AHV Booster Powder twice daily as top dress to your ration. The dosage for this is 100 grams per cow. In this way, Booster Powder provides an immediate energy boost and it stimulates appetite. This ensures that the cow has extra, quickly available energy and she is stimulated to take in even more energy through the feed.

Reactive use of Booster Powder

When health issues occur, Booster Products can also be (re)actively used to provide the cow with quickly available energy. AHV Booster Powder be used as a drench. To do so, mix 1 kilogram of Booster Powder with 20 Liters of lukewarm water. This drench promotes rumen buffering and filling. It also provides an immediate energy boost and hydration. If drenching is not an option, it is also possible to administer two Booster Pastes.

Products AHV Energy & Metabolism Program

14 days before calving and 7 days after calving

Booster Tablet

For reactive use and rapid energy

Booster Paste

As top dress or drench powder

Booster Powder

For reactive use

Aspi Liquid

Sources

Aggarwal, A., & Upadhyay, R. (2013). Hittestress en dierlijke productiviteit (Vol. 188). Delhi, India:: Springer.

Byskov, M. V., E. Nadeau, B. E. O. Johansson, en P. Nørgaard. 2015. Variaties in automatisch geregistreerde

herkauwtijd zoals verklaard door variaties in de opname van voedingsfracties en melkproductie, en variatie tussen koeien J. Dairy Sci. 98:3926-3937.

Florit, E., Romanzin, A., Corazzin, M., & Spanghero, M. (2023). Etenstijd van melkkoeien: een studie gericht op commerciële bedrijven. Italiaans Tijdschrift voor Dierwetenschappen, 22(1), 1023-1032. https://doi.org/10.1080/1828051X.2023.2260827

Kempenaar, C., & van Dooren, H. J. C. (2003). Geen koudestress op lagekostenbedrijf. Praktijkkompas. Rundvee, 17(4), 32-33.

Paudyal S. (2021). Het gebruik van rumination tijd om de gezondheid en reproductie in melkvee te beheren: een review. The veterinary quarterly, 41(1), 292-300. https://doi.org/10.1080/01652176.2021.1987581

Timmerman, M., Van Reenen, K., Holster, H., & Evers, A. (2018). Verkennende studie naar hittestress bij melkvee tijdens weidegang in gematigde klimaatstreken (No. 1117). Wageningen UR Livestock Research.

Van Laer, E., C. P. H. Moons, B. Ampe, B. Sonck, L. Vandaele, S. De Campeneere, en F. A. M. Tuyttens. 2015a. Effect van zomerse omstandigheden en schaduw op gedragsindicatoren van thermisch ongemak bij Holstein melkvee en Belgisch Blauw vleesvee op de wei. Animal 9 (9):1536-1546. doi 10.1017/s1751731115000804

Zom, R. 2016. Hittemanagementstrategieën op melkveebedrijven. Wageningen Livestock Research.