Dr. Dimcho Djouvinov
TECHNICAL MANAGER
AB VISTA
Global modelling predicts $39.94 billion annual losses from heat stress by the end of the century under a high emissions scenario, but with heat stress already having the potential to cost $1.5 billion annually in the US dairy sector, what can Europe do as it faces similar losses in yield, fertility, feed efficiency and health that are expected with rising Temperature Humidity Index (THI). This article explores the risk factors and presents actionable strategies to help producers to protect their margins against the heat.
What is the true cost of heat stress to ruminant producers? If you go from global modelling, it suggests that the economic impact looks set to intensify significantly over the coming decades. Under a high emissions scenario, cattle production losses from heat stress are projected to reach $39.94 billion dollars per year by the end of the century.1
While that may seem a long way off, the growing financial impact of heat stress on producers is not. In the United States, heat stress is already estimated to cost the dairy sector around $1.5 billion dollars annually through lost production and poorer reproduction.2
Across the pond, as Europe experiences more days when the Temperature Humidity Index (THI) exceeds critical thresholds, producers face rising risks to both animal performance and profit margins.
THE REAL IMPACT OF HEAT STRESS
The threshold for heat stress in dairy cattle is generally accepted to fall around THI 68 to 70, depending on breed and physiological status.3 Recent modelling shows that nearly 80% of the global cattle population is already exposed to conditions exceeding this level for at least 30 days per year, and that future climate scenarios will push more regions into prolonged periods of heat load.4 Even in temperate climates, sustained increases in humidity and summer temperatures can result in recurring production losses.
Research across Europe shows that heat stress depresses dry matter intake, reduces milk yield and affects both reproductive efficiency and immune competence. A 2024 prospective cohort study published in Nature showed that increasing THI values in the close‑up dry period reduced subsequent milk production by up to 2.9 kg per day and delayed key fertility indicators, including first oestrus and calving interval.5 Such data reinforces the importance of managing heat stress throughout the production cycle, not only during peak lactation.
PERFORMANCE AND ECONOMIC CONSEQUENCES
It is well established that heat stress lowers milk yield and alters milk composition, but feed intake depression accounts for only part of the problem.
Metabolic research from Cornell University indicates that reduced consumption explains just 30% to 50% of the decline in milk output during hot periods.6 Raised temperatures directly affect liver metabolism, mitochondrial activity, and inflammatory responses, which degrade milk protein synthesis and the overall yield. This complex response to heat stress underlines the need for multifaceted support strategies.
For European producers, these impacts translate into higher production costs and reduced returns per cow. Fertility challenges prolong calving intervals, while health disorders linked to heat stress increase veterinary costs and reduce longevity. As THI days increase across northern Europe, previously low‑risk regions may also see reduced seasonal resilience, with implications for both forage utilisation and feed efficiency.
So, how can European producers mitigate these challenges?
PROACTIVE HEAT STRESS MANAGEMENT STRATEGIES
The first line of defence should always be proactive management. Shading, improved barn ventilation, and the intentional use of fans, sprinklers, and soakers can considerably reduce the heat load on cows. Although the effectiveness varies by temperature and humidity, studies show that investment in cooling systems pays back rapidly through stabilised yield and reduced health issues.7 Even simple measures such as increasing shaded resting areas or improving air movement across feed passages can significantly improve cow comfort.
Maintaining water availability is equally important. Heat-stressed cows can increase water intake by up to 50%. Ensuring clean, cool water that is accessible at all times reduces the physiological strain of thermoregulation and supports feed intake during hot periods.
Feeding practices should be modified to decrease the heat generated during digestion. Producers should consider increasing the proportion of the ration fed during cooler hours to help maintain intakes, while providing smaller and more frequent meals reduces the heat produced during rumen fermentation.
SUPPORTING NUTRITIONAL RESILIENCE DURING HEAT STRESS
Nutrition plays an essential role in helping cows cope with heat stress. Lower fibre digestibility and shifts in rumen microbial populations during hot weather can increase the risk of sub‑acute rumen acidosis (SARA) – a digestive disorder in dairy cows caused by the rumen pH dropping below the healthy range.
Using high-quality, highly digestible forages reduces the heat increment linked with fermentation and helps establish consistent intake. Incorporating rumen‑protected fats can increase dietary energy density without raising metabolic heat production.
Live yeast supplementation (Saccharomyces cerevisiae) is a well-established nutritional strategy for supporting ruminant performance under heat stress conditions. It can enhance rumen function by stabilising pH, promoting fibre digestion, and converting lactic acid to propionate – supporting energy supply during periods when intake is compromised.
LIVE YEAST SUPPLEMENTATION
To counteract the negative effects of heat stress on ruminal efficiency, health and metabolism and to maintain health status, fertility and performance, live yeast has successfully been used in dairy cow diets during periods of heat stress, helping to:
Decrease oxygen in the rumen, improving the conditions for growth of bacteria that convert lactic acid to propionic acid, a major energy source for ruminants.
Effectively compete with starch-degrading bacteria for sugars and reduce their growth and subsequent lactic acid production.
Prevent the accumulation of lactic acid in the rumen, regulating the rumen pH and limiting the risk of both clinical and subclinical acidosis.
For example, herds receiving a double dose of live yeast (100x109 CFU/head/day) during the European heatwave in the summer of 2019 experienced no significant decline in milk yield and showed minimal daily fluctuations.8
Heat stress can increase urinary bicarbonate losses, reducing the amount of bicarbonate available to buffer acids in the rumen. With this decline, ruminal pH can fall, increasing the risk of SARA.
Continuous monitoring of ruminal pH for a 24-hour period showed that animals receiving daily live yeast supplementation maintained a higher pH compared with non-supplemented animals (Figure 1).9
By strengthening rumen function and resilience, live yeast supports feed efficiency, stabilises production and helps protect fertility during extended periods of heat load.
HYDROLYSED YEAST SUPPLEMENTATION
As an additional tool to help mitigate the effects of heat stress, hydrolysed yeast can be incorporated into the diet. Hydrolysed yeast delivers a range of bioactive compounds (β-glucans, mannans, peptides and nucleotides) that support immune function, gut integrity, and the animal’s ability to cope with physiological stress.
Hydrolysis breaks down dead yeast cells into different active compounds, each with their own structure and characteristics. Yeast cell walls become a source of mannooligosaccharides (MOS) and β-glucans while the cell extracts yield peptides and nucleotides. When these compounds are not separated and isolated following the hydrolysis process, they remain together in the final product, retaining their individual functional properties when fed to animals.
The beneficial effect of hydrolysed yeast, fed at doses of 15 g/head/day, was observed in hot season in a heard of milking cows. Milk production of the animals was monitored before and during supplementation, and again after the hydrolysed yeast was removed from the diet, to assess the impact of the feeding strategy (Figure 2).
Figure 2. Effect of feeding hydrolysed yeast (HY) on daily milk yield of dairy cows
Supplementing the diet with hydrolysed yeast resulted in an increase in average milk yield from 35.1 kg/head/day before application to 37.7 kg/head/day after seven weeks of feeding, with a temporary drop observed due to dietary change. Two weeks after the removal of the hydrolysed yeast, milk production was reduced by 2 kg/head/day.
When combined with wider management and nutritional adjustments, both live and hydrolysed yeast products form a key part of targeted mitigation strategies on European dairy farms. Together, they can target different but connected aspects of the heat stress response – rumen function on one side, and systemic resilience on the other.
PROTECTING MARGINS IN A WARMING CLIMATE
As THI days continue to rise across Europe, heat stress is a growing problem, no longer limited to hot climates. Safeguarding herd performance will require a combination of environmental management, appropriate nutritional strategies and proactive monitoring of at‑risk groups such as dry cows. With global modelling indicating substantial future economic losses, early adoption of science‑backed interventions will be essential to protect productivity and maintain competitiveness.
A clear understanding of the specific risks within individual systems, combined with targeted nutrition and mitigation strategies, will be key to limiting the impact of heat stress and safeguarding margins across Europe’s ruminant farming systems in the years ahead.
1. Impacts of heat stress on global cattle production during the 21st century: a modelling study - The Lancet Planetary Health
2. Decoding the dairy dilemma of heat stress | CALS
1. Impacts of heat stress on global cattle production during the 21st century: a modelling study - The Lancet Planetary Health
2. Decoding the dairy dilemma of heat stress | CALS
3. The Effects of Heat Stress on Dairy Cattle Development, Health, and Performance | Veterinary Medicine Extension | Washington State University
4. Global risk of heat stress to cattle from climate change
5. Impact of heat stress during close-up dry period on performance, fertility and immunometabolic blood indices of dairy cows: prospective cohort study | Scientific Reports
6. Decoding the dairy dilemma of heat stress | CALS
7. Dairy cows in distress: New study shows extreme heat is shrinking milk production | Euronews
8. Strategies to mitigate heat stress in dairy cows | The most important additive is intelligence
9. The effect of feeding live yeast cultures on ruminal pH and redox potential in dry cows as continuously measured by a new wireless device