The heat’s UP!

Compared to cattle, any dog suffering the “dog days of summer” has it good. Think about it. Cattle have an on-board fermentation system generating tremendous heat no matter what the weather!

The most graphic “heat stress” incidence I’ve ever seen was on a brutally hot, humid mid-August day with very little air movement. A black 2-year-old bull on pasture started showing classic heat stress signs: panting, staggering, slobbering and trembling.

Even though we moved him to shade immediately, he collapsed and died. Opened up at necropsy, he was literally “cooked” from the inside out. Even his major muscles were as brown as a well-done steak.

Granted, something was wrong with his “thermostat.” The 10 heifers also in the pasture were stressed and uncomfortable, but easily survived the day. While such extremes are rare, more subtle performance losses often occur, and can markedly impact profitability.

Key Points

• Cattle have “rumenal furnaces” generating extra hot-weather heat.

• Deaths tend to occur three to four days into a heat wave.

• Cooling options can help reduce embryo losses.

Assess these risk factors

These eight factors increase the likelihood that a herd will experience heat-related problems:

Heat wave intensity and duration: Most cattle will have no problem with a day or two of high temperatures and humidity. But after several days, they struggle to adapt.

Effects build up. Most deaths occur on day three or four of a heat wave. The National Weather Service calculates a temperature-humidity index warning of potential problems. The “danger” index level occurs at various combinations of humidity and temperatures above 85 degrees. The “emergency” index level begins at 84, and typically occurs in 90- to 100-degree F heat and high humidity.

Hide color: No surprise; dark-hided black or red cattle absorb more heat and suffer more than light-hided cattle.

Weight: Heavy cattle suffer more than light cattle — just like humans. Feedlot cattle close to market weight are at highest risk.

Condition: Just as obese humans mind the heat more than skinny types, fatter cattle are more insulated to heat loss and have greater problems than trimmer cattle.

Health status: Animals with previous health problems are at greater risk, particularly those with respiratory disease. They often have compromised lungs and an inability to dissipate body heat efficiently.

Housing conditions: Confined cattle are at higher risk, since airflow is reduced and concrete floors radiate more heat than pasture grass.

Parasites: Biting flies are perhaps most troublesome. Severe internal parasitism resulting in blood loss also compromises the animal’s ability to cope.

Activity: Cattle that are forced to move excessively, either to seek water or feed, or due to chute work or transport, will “heat up” faster.

Heat ‘chills’ pregnancies

Hot summer temperatures can slam conception rates. An Oklahoma State University study by B.G. Biggers quantified the early effects of heat stress on pregnancy rates.

Cows were exposed to bulls as a group during a period of no stress, then were broken into three groups where two groups suffered through moderate and severe temperatures. The percentage of cows maintaining their pregnancies dropped from 83% in those suffering no heat stress to 50% in those suffering severe heat stress.

Clearly, it’s tough for a new embryo to survive extreme heat. Keep in mind that there’s likely a continuum of negative effects depending on the degree of heat stress. But even a 5% to 10% reduction in pregnancy rate significantly affects your bottom line.

Harpster is a Penn State animal scientist and a beef cow-calf producer.

Don’t be in a pickle over heat stress

If working your cattle is a must, get up early or stay up late to move, work or haul cattle, so temperatures and sunlight intensity are moderated. You won’t suffer as much, either!

Provide plenty of cool water and plenty of space for all animals to drink at will. Temperatures climbing from 70 to 90 degrees will easily double their water consumption.

Provide shade. Quietly moving cattle to pastures with lots of natural shade can provide significant relief.

• In extreme cases, misting or sprinkling cattle can help alleviate stress. But animals will adapt to the wetting; it must be kept up until the heat wave breaks.

Wet down dirt lots. Cattle will seek out wet spots to stand in. It seems to provide a surprising amount of relief.

Alter your feeding program. In feedlot situations, it may pay to feed several smaller meals throughout the day since this tends to decrease the overall “heat of digestion” generated by the animals.

If once-a-day feeding is a must, do it after the day’s peak heat has passed. Decreasing the ration energy level is also effective.

Minimize pests. Flies that “worry” cattle and keep them in constant motion trying to escape the pests only intensify heat stress. Cattle tend to “bunch up” when attacked by biting flies, magnifying the problem.


This article published in the August, 2010 edition of AMERICAN AGRICULTURIST.

All rights reserved. Copyright Farm Progress Cos. 2010.