Heat Stress Tough on Cows This Summer

August 23, 2010 11:41 AM

In many places across the nation, this has been one of the hottest summers in recent memory – and a good test of heat abatement systems.

By Rick Lundquist
My June AgWeb column discussed a simple method to determine body temperatures and heat stress in cows by counting respiratory rates. Since then, I decided to take my own informal survey of respiratory rates on herds around the country. This summer has been a good test of heat abatement systems. It’s one of the hottest summers across the entire country that I can remember.
Here’s a quick reference relating respiratory rate to body temperature based on research done at the University of Arizona.
 Respiratory rate (breaths/min)        Body Temperature (degrees F)
                       45 (normal)                           ​ 101.5 (normal)
                       60                                       ​;     102.2 (mild heat stress)
                       70                                            102.8 (moderate heat stress)
                       80                            &​nbsp;               103.2 (high heat stress)
                     100                         ​                   104.0 (severe heat stress)    
The first week of August was typical Florida weather, with highs in the 90s and relative humidity in the 80s. I found many cows in Florida with respiratory rates of 100, equating to a body temperature of 104 F.  These cows were in herds where there was little heat abatement or had cooling systems that were not operating properly. Fans alone did nothing to reduce respiratory rates and, hence, body temperature in the heat and humidity of Florida. With fans and soakers operating properly, I counted respiratory rates of about 60, indicating relatively good heat abatement for this climate.
Last week in Arizona, temperatures were in the low 100s, with relative humidity near 30%. Again, respiratory rates depended on the extent of heat abatement on the dairy. When cows had just shades (no fans, sprinklers or misters), respiratory rates were in the 70s-to-80 breaths/minute. Cows with good fan coverage and misters in Saudi style barns had respiratory rates of 50 – 60, with some cows near normal.
This illustrates not only the effectiveness of heat abatement systems, but also the effect of humidity on the ability to cool cows. Typical Arizona dry heat is much easier to deal with than the humidity in the Eastern U.S. However, the monsoonal weather in Arizona during July and August, with temperatures over 110 and high relative humidity, really caused some dairies to hit the skids on intake and production.
Last week was relatively hot and humid in Wisconsin too, with highs near 80 F but humidity also in the 80s. Here again, humidity affected cows even with good heat abatement. Cows had respiratory rates of about 60, even with excellent fan and sprinkler coverage.
Feed intake and milk production losses on these particular herds were pretty well correlated with respiratory rates that I observed on these visits. For whatever my informal survey is worth, I’m convinced that every dollar invested in heat abatement systems and proper maintenance of these systems will pay dividends.
Rick Lunquist is an independent nutrition and management consultant based in Duluth, Minn. He provides livestock production advice.

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