Global warming, with new heat records being set each year, has brought the issue of heat stress into sharp focus—not just for humans, but also for dairy cattle. High temperatures negatively impact the performance, health, and fertility of cows and calves. It is now understood that heat stress during late gestation negatively impacts the performance and health of offspring throughout their lives. Scientific research suggests that the effects of heat stress can be observed several generations later.
Cows like it cool, and fare best at temperatures ranging from -5 to 15°C. With extreme weather conditions becoming more common in Europe, heat stress in dairy cows is now a frequent issue. Additionally, the problem is compounded by the fact that average milk yield has increased, which also increases body heat production due to higher metabolic rates. Our high-performance dairy cows therefore have to contend with a double physical burden. Like most mammals, cows have very few sweat pores and can hardly sweat at all. This makes it more difficult for cows to get rid of excessive heat. In warm temperatures, cows can only thermoregulate by breathing, minimizing activity and radiating heat through their body.
Research shows that cows suffer from heat stress faster than is commonly believed. Heat stress is caused by a combination of temperature and relative humidity: the Temperature Humidity Index (THI).
As illustrated in the figure, cows may start experiencing heat stress when the THI surpasses 72, leading to adverse effects on fertility. As the THI climbs above 78, milk production is substantially affected, resulting in decreased yields. When the THI rises beyond 82, the situation becomes critical. Cows will begin to exhibit signs of severe stress, such as increased respiration and decreased feed intake. In this case there is a risk of significant losses in milk production, and in extreme cases, cows may succumb to the stress.
The average relative humidity in the UK is 60% in the summer months. At this relative humidity, heat stress occurs as early as 22°C.
icons below to find out more.
🡑 Incubation & toxin formation
🡓 Palatability
🡓 Dry matter intake
🡑 Water absorption
Water consumption will initially increase, but as heat stress worsens, cows may stop drinking altogether.
↑ Breathing (>30/minute)
↑ Salivating
↓ Rumen activity (< 10 contractions/5 min)
↓ Rumen pH (rumen acidification)
The cow will eat less, which will reduce rumen function. As a result, rumen acidification may occur.
🡓 Fertility
Heat stress in summer can cause deteriorating fertility in autumn.
🡓 Milk production
🡓 Milk fat and protein levels
🡑 Somatic cell count
A direct indicator of heat stress is reduced milk production. In high producing cows (50 kg milk/day), heat stress can lead to a decrease in daily production by as much as 6 kg of milk. Milk fat and milk protein levels are also known to decrease (up to 16%) and heat stress is known to increase the risk of increased somatic cell count.
🡓 Claw health
Heat stress in summer can cause a decrease in claw health in autumn.
Increased infection risk: The cow's immunity resistance will decrease due to stress and possible rumen acidification. The combination of high temperatures and humidity creates an environment where pathogens can thrive. This increased infection pressure in the barn can lead to a higher incidence of diseases.
Long-term lower milk production: Heat stress can cause a reduction in milk production that extends beyond the immediate period of stress. Research indicates that heat stress leads to the death of more cells in the udder tissue, which may contribute to long-term decreases in milk yield. For example, cows that experienced mild heat stress in dry heat produced 6.3 kg less milk per day in the subsequent lactation.¹ Additionally, heifers born to heat-stressed cows showed significant reductions in milk production during their first three lactations, with decreases of 2.2 kg/day, 2.3 kg/day, and 6.5 kg/day respectively.²
Impact on dry cows and calves: Pregnant cows and their unborn calves are particularly vulnerable to heat stress. Calves born during or after hot periods tend to be lighter at birth and more prone to illnesses. This early-life stress can have long-term effects, resulting in poorer milk production and shorter lifespans. To mitigate these effects, it's crucial to ensure that dry cow areas are well-ventilated, for example, by using fans, to provide a cooler and more comfortable environment for both the cows and their developing calves.
Receive valuable insights in your inbox
Ready to learn more about how to combat heat stress in your dairy herd over the course of the summer months? Fill out the form below with your first name, last name, and email address to receive valuable information and tips directly to your inbox. Stay ahead of the heat and keep your cows healthy and productive!
With our THI calculator, you can easily determine if your animals are experiencing heat stress and discover what actions you can take to reduce it. Get instant recommendations to ensure your animals stay healthy and comfortable.
Citations:
1. Tao, S., Monteiro, A. P. A., Thompson, I. M., Hayen, M. J., & Dahl, G. E. (2012). Effect of late-gestation maternal heat stress on growth and immune function of dairy calves. Journal of Dairy Science, 95(12), 7128–7136. https://doi.org/10.3168/jds.2012-5697
2. Laporta, J., Ferreira, F. C., Ouellet, V., Dado-Senn, B., Almeida, A. K., De Vries, A., & Dahl, G. E. (2020). Late-gestation heat stress impairs daughter and granddaughter lifetime performance. Journal of Dairy Science, 103(8), 7555–7568. https://doi.org/10.3168/jds.2020-18154