By Matt Dodd, DVM / Ruminant Field Technical Specialist

CEDAR RAPIDS, Iowa — From the ripple effect in future generations to how cows “heat up” over the summer, experts are still learning insights on how cows handle and react to heat stress. Here are five things to know about heat stress, along with practical applications for mitigation and supporting cow comfort as heat and humidity rise.

The number of “heat stress days” are increasing across the U.S.

A heat stress day is when the combination of temperature, humidity, and duration of heat exposure pushes cows beyond their comfort zone, leading to physiological stress and potentially lower production. Cows can start experiencing heat stress when the temperature-humidity index (THI) reaches 68 degrees Fahrenheit or higher, which can happen even in the low 70s with moderate humidity.

Heat stress can damage cows on a cellular level. Heat stress can dehydrate cells and reduce temperature regulation. Heat stress can also throw off the balance between a cow’s natural defense system—antioxidants—and harmful molecules—free radicals. Once a cow’s immune system is off-balance, the door is open for poor outcomes like slower recovery, decreased milk production, and reproductive health challenges.

Proper hydration is necessary to combat cell damage from heat stress. To stay hydrated, cows need to drink ample amounts of water. However, drinking water alone may not provide the relief and nutrients they need. Ration adjustments, such as lowering energy density, or incorporating a heat mitigation additive can work inside the body to lower temperature and balance cell hydration. Heat mitigation products containing a blend of ingredients, such as antioxidants, vasodilators and osmolytes for example, have been shown to lessen the heat’s impact on milk production while also improving first-service conception rates.

Adjusting nutrition and feeding practices make an impact.

H2O – Increasing water consumption is the single most important part of your nutritional strategy because cows need water to release heat and control their body temperature. Keep waters clean and add more opportunities for cattle to drink while going to and from the parlor.

Feed at night – Cows will avoid eating during the hottest hours of the day. In fact, cows can consume 2/3 of their diet during cooler nighttime temperatures. If feeding one time daily, try to feed at night. If feeding cows twice or more daily, feed 70-80% of their daily allowance at night.

Add stabilizers to the mixer wagon – Because yeast and mold favor heat, warmer outside temperatures can cause fresh feed to spoil faster. A three-acid blend of acetic, benzoic, and propionic acids is ideal to control yeast populations and protect rations against mold and mildew.

The impact of heat stress lasts generations.  Research from the University of Florida and the University of Georgia show that mitigating heat stress in pregnant dairy cattle is good for the health of their offspring and next generation.

Key findings include:

• Growth and development: Calves born to cows affected by heat stress during late lactation tend to be smaller and have stunted mammary gland development.

• Health risks: Calves born to heat-stressed cows have higher morbidity and mortality rates due to a lower amount of passive immunity in colostrum being recorded at birth.

• Long-Term Productivity: A 10-year dataset comparison revealed that dry cow heat stress ripples beyond cow impact and can have biological effects on the next two generations.

Based on these findings, investing in heat abatement equipment for dry cow housing such as shades, fans, and soakers merits strong consideration. Additionally, nutritional heat mitigation support can be considered as long as it’s safe for dry cow consumption and does not contribute to increasing the DCAD of the diet.

The impact of heat stress is lower earlier in the summer. One simple explanation for this is that barns and concrete take time to absorb heat in the summer after being cold all winter. Once that heat is absorbed, infrastructure can retain and radiate a significant amount of heat onto cattle. For example, you have likely noticed how much heat radiates from streets and buildings if you’ve ever been in a city during the summer.

Keep in mind that reduced dry matter intake is the first sign that heat stress is setting in, followed by:

• Reduced rumen function

• Decreased milk production

• Challenged reproduction

A cow’s body temperature continues to rise for several hours after peak temperatures, often reaching its highest point around 9 p.m. This makes it crucial to continue cooling cattle even after the sun sets.

In conclusion, there are more complexities to heat stress than what first meets the eye. Consider the long-term benefits of focusing physical and nutritional resources on heat stress mitigation and helping your cattle perform throughout the hot summer months. Lean into your team of experts for more guidance on what may help your herd this season.