Tall Grass, Good Cattle, and Heat

We're approaching the hottest days of the year for most of the country. Heat and humidity bring along with it heat stress and heat exhaustion in those who spend a lot of their time outside.

Of course, this is also true for animals, and you are already likely aware of the challenges you’re faced with when trying to protect your outside livestock from the dangers of summer heat.

The North Dakota State University (NDSU) Extension website offers a great resource on dealing with heat stress in beef cattle. You’ll also find a handy chart for heat index, or what the temperature actually feels like against your skin when you take relative humidity into account. Using the chart as reference, a heat index at or above 75°F is a time to take notice and be aware; getting over 76°F could become a dangerous situation for cattle. A heat index over 84°F is an emergency and action should be taken.

While this article from NDSU recommends you pay attention to the weather forecasts to plan ahead, it may also be useful to know approximately how many days you can typically expect these types of conditions to exist. Knowing ahead of the season how many times you’ll normally have to prepare for managing heat stress can help you better prepare.

Let’s first take a look at normal July maximum temperatures across the country. I’ve drawn a red line across the country to denote the 90°F boundary. South of this line, average July high temperatures are over 90°. I’ve also added a purple line to denote humid conditions. Inside the purple area, average July dew point is greater than 60°F. Living within the hot boundary or the humid boundary can make summer uncomfortable. Living within both boundaries likely feels like you’re living inside a sweltering shower stall.

To get more specific, let’s next consider the intervention guidelines listed in the NDSU article. They recommend that you intervene to further protect your cattle in the following 3 instances:

When the daytime heat index is greater than 84°F for
48 Consecutive Hours
Daytime heat index greater than 79°F and nighttime is greater than 75°F for
48 Consecutive Hours
When the heat index is greater than 75°F for 72 consecutive hours
72 Consecutive Hours

The following maps were created for June, July, August, and the warm season (defined as April through September). A dangerous day meets one of the thresholds listed above by considering normal hourly heat index values (normal, or the average, defined for the 1981-2010 time period). For July, the hottest month of the year, red stations on average experience these dangerous days for the entire month. These dangerous days extend as far north as I-80 (in the Midwest, largely driven by humidity) and across the entire desert Southwest (mostly driven by very hot temperatures). Orange colored stations may see dangerous conditions on almost half the days in July. And yellow colored stations will typically experience less than 2 weeks of dangerous days.

Number of Dangerous Heat Index Days

When considering the warm season, areas of south Texas, southern Arizona, and south Florida can experience dangerous days, with little to no reprieve, for the entire season! Ideally, you’re in the best situation if you live near an area with a yellow or orange dot… although things obviously change when you switch over to the cold season. 

The key takeaway here – unless you live in an area near a grey dot (where, on average, zero days meet the thresholds listed above), heat index shouldn’t be too far from your thinking during the summer.

How does this summer compare to average?

So far, this summer has been cooler than average for most of cattle country.  The map below shows the Cattle Comfort Index (developed at the University of Nebraska) anomaly, or difference from average, for May through July.  The Index was specifically developed for livestock and is interesting because it is a heaat index except that it takes into account more things than just average temperatures and relative humidity.  The Index also incorporated wind speed and solar radiation, which gives a more complete picture of what’s going on.

Going from west to east: California and the Pacific Northwest have seen warmer than average conditions.  So too have South Texas, the Midwest, and the Southeast up to the East Coast.  Everything in between has been cooler than average.

Cattle Comfort Index Map: May-July 2019

If you’re curious what this looks like for total cattle numbers in cold or heat stress we break this down below, in the two charts.  The chart on the left shows cattle by state and how much the Comfort Index is higher/lower than the summer average.  Red is warmer than average and blue is cooler than average.  Just eyeballing it you can see way more cattle are sitting in cooler conditions this summer.  If you would like to see the actual numbers the chart on the right shows over 50 million head have observed cooler than average conditions while approximately 30 million are seeing warmer than average conditions.  Hopefully that will translate into good gain for cattle grazing and those being fed in feedlots. 

We’re just getting into the heart of summer, though, so we have a long way to go.

Cattle in Heat Stress this Summer: State Breakdown

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