Effects of heat stress on dairy cows ‘expected to intensify’

In the US, heat waves in the last 60 years have become more frequent and intense. Today, the US experiences around three times as many heat waves than in the 1960s; the average heat wave season lasts around 46 days longer, and temperatures hover about 2.5°F (1.3°C) above the local threshold.

Spring temperatures in particular have increased by about 2°F (1.2°C) compared to a 1.6°F (0.8°C) in the summer and fall, suggesting it’s now also hotter during the peak dairy production season. At the same time, major dairy states in the North East and the South West have experienced some of the highest rates of temperature changes​.
We spoke to two experts to find out what the future holds for dairy producers tackling heat stress in their dairy herds – and what resources are available.

Peri Rosenstein is a veterinarian and senior scientist, Livestock Systems, at the Environmental Defense Fund.

Ananda Fontoura is a veterinarian and Postdoctoral Associate, Animal Science at Cornell University and expert on environmental physiology and nutrition.

DR: How are longer and more severe bouts of heat affecting cow health in the last decade or so? What are some of your key observations with regards to heat stress in cows in recent years?​

Peri Rosenstein:​ Heat stress can reduce milk production, reduce dry matter intake (only accounts for a portion of the decrease in milk yield that is seen), reduce reproductive efficiency, increase morbidity, and compromise the general welfare of cows. With the combination of increasingly high heat indices and the high metabolic rate of current dairy cows, these responses are further exacerbated.

Ananda Fontoura:​ In recent years, longer and more severe bouts of heat stress have significantly impacted the health of dairy cows.

It is well documented and researched that exposure to high ambient temperatures triggers various behavioral adaptations in dairy cows aimed at reducing heat load and maintaining comfort.

These animals adjust their physical activity and resting patterns to seek shade and increase heat dissipation by lying down less.

Physiologically, cows exposed to high ambient temperatures exhibit increased body temperature and respiratory rates as immediate responses to heat stress. There is notably a decrease in feed intake, which is crucial as cows attempt to minimize metabolic heat production. This reduction in feed intake not only affects their energy levels, but also has implications for overall milk production, reproduction, and health.

Importantly, recent research has confirmed the hypothesis that heat stress compromises gastrointestinal barrier function, allowing harmful bacterial products such as endotoxin to translocate into the bloodstream. This physiological challenge further exacerbates the overall stress response and can impact health outcomes for dairy cows, complicating the prognosis in each case.

DR: What regions of the US have been the hardest hit when it comes to heat stress in dairy? What indications are there that heat stress is starting to affect more producers in states that weren’t as affected before?​

AF:​ Historically, states in the South, including (but not limited to) Florida, Texas, and Arizona, are hit the hardest when it comes to heat stress. However, predictions estimate up to 1.5°C (2.7°F) increase in temperatures in cooler areas. This also become a huge concern because in already hot areas this can increase up to 5°C (9°F).

PR:​ The impacts are seen in the beef industry as well. Last summer, hundreds of beef cattle died in Iowa, Kansas, and Nebraska as a result of the high heat index.

DR: What are climate models projecting in terms of how warming is set to affect US dairy in the coming years?​

PR:​ Climate models demonstrate that we need significant reductions in carbon dioxide and methane emissions to slow the rate of warming over the next few decades. In the meantime, we are seeing a progressive rise in global temperatures that will cause continued impacts of heat stress and other climate-related stressors on animals and the environment that will affect the US dairy industry.

AF:​ A 2021 report by the IPCC​ predicts that by 2100, global temperatures may increase up to 4.8°C / 8.8°F) if greenhouse gas emissions are not reduced. Alongside rising temperatures, there is also a trend of more frequent and severe extreme heat events, coupled with a decline in extreme cold events across North America.

“The adverse effects of heat stress are expected to intensify over time, which is particularly concerning for US dairy farmers, as high-producing dairy cows are more vulnerable to the impacts of a warmer climate considering their high metabolic rate and milk production capacity.”

DR: What are the financial implications of heat stress in dairy cows – both in terms of milk losses but also in preparedness costs for producers?​

AF:​ In the US, heat stress creates a substantial economic challenge for the dairy industry, costing around $1 to $2.5 billion each year. Forecasts suggest that this financial impact might grow by an extra $126 million over the next 50 years. The economic losses are attributed to reduced milk yields, increased pregnancy loss rates, higher disease prevalence, and, in severe cases, the deaths of dairy cattle.

DR: How can industry, government and producers address the challenges posed by extreme heat to the dairy industry?​

PR:​ Producers will have to continue to evolve to manage heat stress and adapt to other impacts of climate change. We also need support for additional research on heat stress mitigation strategies.

DR: How effective are traditional methods of coping with heat stress in dairy cows in today’s increasingly volatile weather? What should producers know in order to be better prepared to react to extreme weather events?​

AF:​ Conventional approaches aimed at mitigating the harmful impacts of heat stress focus on implementing heat reduction methods such as shading, fans, and water sprinkler systems. And although these measures are very effective, they are also quite expensive and require significant resources such as energy and water, which can be hard to access in certain areas of the world.

“Due to the ongoing challenges posed by heat stress to cow health, welfare, fertility, and milk production, there is a pressing need to explore alternative strategies that enable heat-stressed animals to sustain their well-being and productivity.”

PR:​ There are a variety of interventions that can be deployed, including ventilation, cooling tools such as fans and sprays, limiting overstocking, and changes in feed management and diet formulation.

DR: What is the outlook for this summer’s effect on milk production, particularly with regards to heat stress in dairy and other factors such as the bird flu outbreak?​

PR:​ It is largely unpredictable – there is typical summer variation, effects of global record temperature highs, and additional factors including disease outbreaks and the response to them that can all impact productivity. While these factors usually cause reductions in milk yield, the magnitude can be highly variable.

How is US milk production tracking so far this summer?According to USDA data, milk production in the 24 major states in June was down 0.8% year-on-year. Production per cow averaged 2,025lb for June, 8lb down year-on-year.

The 24 states are Arizona, California, Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Pennsylvania, South Dakota, Texas, Utah, Vermont, Virginia, Washington and Wisconsin.

The next USDA update is due in August 2024.