Modern dairy cows live within a surprisingly narrow comfort zone, thriving between 5°C and 25°C. By comparison, humans feel comfortable at temperatures up to 26°C, which means we often underestimate the stress cows feel as temperatures rise. As heatwaves become more frequent, managing heat stress has become a central pillar of sustainable dairy farming.
Thank you for reading this post, don't forget to subscribe!Heat stress quickly alters cows’ behaviour and metabolism once temperatures exceed their comfort zone. Feed intake decreases to limit metabolic heat, which in turn slows rumination, destabilises the rumen and reduces milk production.
Reproduction also declines due to hormonal disruptions, weakened embryo development and lower conception rates, which can persist for months. Impaired immune function, oxidative stress, and inflammation induced by heat stress further reduce production and make cows more prone to health disorders. Together, these metabolic, behavioural, immune, and fertility impacts compound, making heat stress a major threat to overall productivity.
The Hidden Bill of Heat Stress
While the immediate impact of heat stress on milk production is already concerning, economic analyses reveal a much larger financial burden. Modelling from the USA shows that heat stress costs the dairy industry between US$897 million and US$1.5 billion per year, depending on the level of cooling used. These losses reflect increased costs and reduced profits due to lower milk income, slower growth, increased mortality, lower fertility, increased mastitis incidence, premature culling and higher veterinary costs.
However, recent research has shed light on a far more insidious and costly consequence of heat stress what happens when a cow experiences high temperatures during the dry period. Dry cows often do not receive the same level of cooling resources as lactating cows since they are not producing milk. Yet heat stress during this time affects the dam’s mammary gland redevelopment, reducing the number of secretory cells and leading to lower milk production in her next lactation.
Not only do heat-stressed dry cows produce less milk, but their daughters are also impacted. A calf born from a heat stressed pregnancy is disadvantaged from the start and struggles to reach its full productive potential. Those calves are typically smaller, grow more slowly and display signs of reduced immune competence. Their mammary gland development is also impaired, resulting in reduced milk production throughout their lives. On average, they produced 2.2, 2.3 and 6.5 kg less per day in their first three lactations, respectively. Those daughters also had shorter productive lives, being culled five months earlier and requiring replacement sooner.
What makes this phenomenon even more critical is that the influence of in utero heat stress extends to a third generation granddaughters arising from oocytes that developed inside the foetal daughter are also affected. Changes in mammary physiology are still evident in these animals and are again associated with decreased milk production. Indeed, the researchers reported that granddaughters of heat stressed dams produced 1.3 kg less milk per day during their first lactation than granddaughters of cows that were cooled. These granddaughters were also removed from the herd earlier, resulting in a 32% reduction in longevity. This means that heat stress experienced by a dry cow results in multigenerational consequences, shaping not only the calf she carries but also the calf’s future offspring.
Farmers rarely see these generational effects immediately, which is why the dry cow period is often underestimated. The cow may look outwardly stable, yet lasting damage is unfolding internally, only to be revealed months or years later as lower milk yields and weaker youngstock. These hidden effects add further to the economic losses associated with heat stress. Annual US losses due to in utero heat stress alone are as high as US$595 million. For dairy farmers worldwide, these findings highlight the enormous financial benefit of protecting dry cows from heat stress.
Strategies for Heat Stress Resilience
Cooling infrastructure is essential for protecting herds, but nutrition also plays a powerful role in helping cows cope with heat stress. Strategic supplementation of essential nutrients allows cows to maintain metabolic balance, support immune resilience and protect mammary tissue despite the pressures of high temperatures.
Research conducted by Adisseo has shown that providing the correct amino acid balance, particularly methionine (Met), helps maintain milk fat and protein levels even during heat stress. Methionine has also been well documented to support more efficient liver function and improve immunometabolism during both heat stress and the transition period. However, one question was whether Met supplementation during transition heat stress could improve the production and health of both the cow and the developing offspring.
To investigate this, researchers fed rumen-protected methionine (Smartamine M +7.6 g metabolizable Met pre- and 10.8 g postpartum) for 6 weeks before and 4 weeks after parturition, while also exposing the cows to heat stress with electric heat blankets for 4 weeks before and after parturition.
- Greater milk protein Supplementation with Met increased milk protein percentage (+0.18%) compared to cows under heat stress without supplemental Met. In fact, Met supplementation restored the milk protein percentage to the same level as that of cows kept under thermoneutral conditions and fed the CON diet.
- Thermoregulation and liver function Cows fed Met also had lower rectal temperatures postpartum than cows fed the CON diet and showed improved liver function, indicating that Met supplementation helped cows better handle the metabolic effects of heat stress.
- Rescued mammary microstructure Heat stress adversely affected mammary secretory structures (fewer alveoli, where milk is produced), whereas Met supplementation mitigated these effects. Methionine also increased mammary mRNA expression of AA transporters and genes regulating Met metabolism and protein synthesis. These improvements in mammary physiology and metabolism most likely supported the greater milk protein synthesis of Met-supplemented cows.
- Enhanced calf growth Maternal methionine supplementation during late gestation heat stress led to taller heifers at birth with lower rectal temperatures, which was linked to a larger placenta with greater AA transfer compared to heifers from unsupplemented dams. Together, these results indicate that integrating targeted nutrition with effective cooling can protect milk yield and strengthen the resilience of future generations.
Tools to Fight Heat Stress
The economic impacts of heat stress are far greater than the visible summer decline in milk. Exposure to heat stress during the dry period results in hidden losses that lead to long lasting effects on the cow, her calf, and even her granddaughter. However, with vigilant heat abatement, improved nutritional strategies such as Met supplementation, and special attention to dry cows, farmers have powerful tools to protect their herds and stabilise productivity. With Adisseo, supporting cows through heat stress leads to healthier animals today and stronger, more productive herds in the years to come.