What is heat stress and what are the consequences?

Energy is fuel to a cow’s body. Available energy is distributed based on priority. If energy intake decreases or energy requirements are allocated to increased health challenges, there are consequences to performance. These consequences can be seen in decreased milk production, decreased growth, poor fertility, and compromised immunity. Hot weather always increase the likelihood of health challenges due to reduced energy levels. At AHV, we like to help you and your livestock get through the warm periods.

When do we speak of heat stress?

Every animal has a thermoneutral zone, the interval of ambient temperature in which it does not expend extra energy to keep its body temperature constant. For cows, this zone is between -14 and 72 degrees Farenheit (°F) (Kempenaar and van Dooren, 2003). Outside this thermoneutral zone, cows consume additional energy to actively warm up or cool down. This is at the expense of milk production, growth, fertility and immunity. Lactating animals are much more susceptible to heat stress than young stock and beef cattle due to their high heat production from digestion of their high feed intake. Fresh cows, high-producing cows and dry cows are at the highest risk because they are not in an optimal energy balance.

Table 1. Temperature Humidity Index (THI) based on the formula used by Van Laer et al. (2015a) with the threshold of Zom (2016) where:

green = no heat stress, yellow = mild heat stress, orange = heat stress, dark orange = severe heat stress, red = lethal heat stress

Heat stress is generally defined as a situation where there is an imbalance between the cow’s heat production and its ability to release this heat to the environment. This happens as soon as the upper critical temperature of the thermoneutral zone is exceeded. To determine whether the environment can cause heat stress in cows, heat stress indicators such as the Temperature Humidity Index (THI), Table 1, are used. The THI uses two important parameters, temperature and relative humidity, to measure the effect of climate on animal response (Aggarwal and Upadhyay, 2013).

Dairy cows experience heat stress at a THI of 68 and up while beef cattle start becoming heat stressed at a THI of 78. In the United States, with the temperate climate and high relative humidity, dairy cattle can already experience mild heat stress as soon as the ambient temperature rises above 70°F (Timmerman et al., 2018). The THI does not consider solar radiation, cooling by wind or duration of heat stress.

How does heat stress affect your dairy cows?

Cows lower their feed intake during hot weather (68°F+) by 10-12% or more. This leads to reduced milk yield (15-40%), poor reproductive performance and a greater incidence of metabolic health challenges (Manitoba, 2023). At temperatures above 65°F, it is important to be alert for possible negative impacts of heat stress because of the high impact on the animal’s general health. The following systems are most affected by heat stress (Timmerman et al., 2018):

• Production
• Immunity
• Rumen function
• Hoof
• Udder health
• Fertility

The effect of heat is not always immediate, and it can sometimes take up to 10 days to show its effects. Therefore, it is important to be proactive during a hot period. This starts with keeping an extra eye on the weather and highest risk animals. This way, it is possible to anticipate the cow’s energy balance in time. This enables the cow to cope better with warm temperatures without compromising her health and production. Listed below are some indicators you need to take into consideration, depending on the temperatures and your cow’s history: 

• Feeding time: We recommend a total feeding time of between 4-9 hours. 
• Ruminating time: We recommend a total ruminating time of between 4-10 hours. 
• Chewing time: We recommend a total chewing time of around 16 hours. 
• Ruminations: We recommend a minimum of 60 ruminations. 

What can AHV products do for your cows?

With heat stress, the feed intake of the cow drops, even though they need extra energy to actively cool down and maintain resistance to health challenges. It is, therefore, very important to provide your cows with extra energy and to stimulate their feed intake and rumination. At expected high temperatures, we recommend feeding AHV Booster Powder as a top dress at the herd level. This is to support health with extra energy and prevent the risk of falling milk yield. The dosage is 50 grams per cow, twice daily.

Wondering how AHV can support your cows during hot periods? Fill in the form below or contact your AHV Farm Advisor for a consultation without any additional service costs.

Sources:

• Aggarwal, A., & Upadhyay, R. (2013). Heat stress and animal productivity (Vol. 188). Delhi, India:: Springer.
• Kempenaar, C., & van Dooren, H. J. C. (2003). Geen koudestress op lagekostenbedrijf. Praktijkkompas. Rundvee, 17(4), 32-33.
• Manitoba, 2023. Feeding Heat Stressed Dairy Cows
• 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 of summer conditions and shade on behavioural indicators of thermal discomfort in Holstein dairy and Belgian Blue beef cattle on pasture. Animal 9 (9):1536-1546. doi 10.1017/s1751731115000804
• Zom, R. 2016. Heat management strategies on dairy farms Wageningen Livestock Research.