Combating heat stress in dairy cattle

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.

When does heat stress occur?

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 Ireland is 60% in the summer months. At this relative humidity, heat stress occurs as early as 22°C.

Recognizing the signals of heat stress in dairy cows

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Feed quality and intake

Fysiology

Reproductive health

Udder

Claws

Impact of heat stress on production and animal welfare

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.

Managing heat stress in dairy cattle

Besides impacting the welfare of dairy cattle, heat stress significantly decreases milk yield and quality, resulting in financial losses. A good heat abatement strategy should include proper adaptations to housing, providing shade, adapting nutrition strategies, and case-specific interventions.

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Stay ahead of heat stress

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THI calculator

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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