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What is the true impact of heat stress?

Mark Carson Published on 30 June 2010

A long, hot summer will impact the health and performance of dairy herds. There are many different systems that can be used to effectively cool cows in the summer. So, instead of focusing on the how to keep your cows cool, I want to discuss some common areas to watch to make sure your cows don’t get overheated and where the impact of heat stress may show up in your herd.

When looking for impacts of heat stress in a herd, I often start by looking at simple stats like conception by month, pregnancy rate and milk production during this period. Though these are good place to start, some herds have the impact of heat stress hidden in different areas. The total effects of heat stress on pregnancy, insemination and conception rate don’t just impact a cow’s performance from June to August – the effects of these hot months can continue to appear from September to December.



It is my belief that some herds that have small drops in conception (~5 percentage points) in the fall are likely due to events that occurred in the summer months. For a 100-cow herd, a 5-point drop in conception for the fall months can cost more than $5,000 in lost potential revenue over the year. When considering the impact of heat stress in your pocketbooks, you may want to look at performance over a large period of time.

Transition cows
Transition cows are the most important cows in your breeding program. Transition for a cow can be stressful enough – adding high temperatures makes things worse. Ensure these cows have everything they need to stay cool. Do not overstock them during this time (80 percent stocking density is recommended), and minimize competition for bunk and stall space.

Keep a close watch on the number of health events: retained placentas (RP), displaced abomasums, ketosis and other events that occur during this period. If the number of health problems peak in your herd during the summer months, it’s worth investigating what is causing these issues to arise. A good transition period for a cow will increase her chances of getting pregnant two to three months late!

Feed quality
Total mixed rations (TMRs) spoil much faster in the summer heat. If your TMRs don’t smell right or look right, it likely doesn’t taste right to your cows either. Spoiling TMRs causes less dry matter intake (DMI) for cows, which will directly impact the performance of your herd. Regularly monitor the transition cows’ rations for quality. If you’re only making your transition cow TMRs every other day, it’s worth making new rations every day during the summer months. Any drop in dry matter intake with the transition cows during the summer months can lead to having several problem breeders in the fall.

Heat stress affects heifers too. With heifers, they are often out of sight, which makes them out of mind. Summer heat not only directly impacts their reproductive performance, but slows the growth of heifers that can follow them into their lactation, impacting their performance. Keep stocking density low, make sure the heifer have access to shade and access to an abundant source of clean water. As mentioned above, if feed quality begins to spoil from the summer heat, look at increasing the frequency of feeding.


The effects of heat stress on your cows may not be obvious or may not appear until after summer is over. Managing your transition cows, feed quality and heifers through the summer heat will help reduce the impact that heat stress will have on your herd during the fall months. PD

Excerpts from Repro-Active Solutions blog

Mark Carson
Reproduction Analyst