More than 1,100 head of feedlot cattle died in July, 2007 in South Dakota because of unfavorable environmental conditions. The combination of high daytime temperatures, high humidity, low wind, minimal daytime cloud cover, and lack of nighttime cooling created the stifling situation.

A University of Nebraska publication by Dr. Terry Mader and others ( addresses cattle crises and possible deaths due to excessive heat and humidity. It’s recommended to focus on the temperature humidity index (THI) chart (see table) to help predict when cattle will get into trouble.

Heat stress in cattle is usually precipitated by a combination of environmental factors:

  • Temperatures in the 80s or above
  • Relative humidity above 30%
  • Overnight temperatures above 70°F with no nighttime cooling
  • Wind speed below 5 mph
  • Lack of cloud cover or shade  —  especially with dark-hided cattle
  • Be especially alert when the combination of above conditions occurs 2 to 3 days or more in a row

Several management factors can make the problem worse:

  • Inadequate fresh, cool water
  • Having to travel long distances to reach water
  • Handling cattle during the heat of the day
  • Fescue toxicity problems

Solutions include making sure cattle have easy access to ample water. Avoid penning or handling cattle whenever possible. If you must handle cattle, do it at sun-up only. In an emergency, local fire departments may be called in to spray cattle.

While losses in South Dakota occurred primarily in heavy feedlot cattle, the same could happen in any well-conditioned cattle, including show cattle. In heat-stress prone situations, advise livestock producers accordingly.

Blue-green algae

Another hot weather livestock problem that may be observed is associated with blue-green algae bloom on ponds or water tanks. This problem usually occurs in shallow, warm, slow-moving or still water that is high in nutrient content. These conditions frequently allow the growth of Cyanobacter spp., which is not an algae at all, but a form of photosynthetic bacteria. When conditions are right there will be massive growth of these bacteria, resulting in what is known as a “bloom”. During a bloom bacteria float to the surface and collect to form what is commonly called pond scum. Wind pushes this floating pond scum across the top of the water, concentrating the scum against downwind shores. Often there is a distinct odor associated with this bloom.

Cyanobacter spp. are capable of producing some highly potent toxins  —  some that affect the liver, some that affect the nervous system, and some that may cause irritation of exposed skin. The toxins produced affect humans as well as most domestic animal species. Some of these toxins are rapidly fatal, and cases of cattle dying within 30 minutes of drinking algae-bloom water have been reported this year. Bulls and heavy milking cows may be the first to die because of their higher total consumption of contaminated water.

If blue-green algae problems are anticipated or suspected, several management options are recommended:

1. Do not wade, swim in or drink water from these water sources

2. Provide alternate sources of drink-ing water for livestock

3. Fence cattle away from the north half of ponds where the downwind accumulation of the bacteria might occur

4. Test pond scum to see if blue-green algae is present. Veterinary diagnostic laboratories and other laboratories can test water samples for producers. 

This information was excerpted from the Kansas Veterinary Quarterly, Summer 2007, Larry C. Hollis, DVM, M. Ag, Extension beef veterinarian, Kansas State University.