By Harold Harpster
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 two-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, they can markedly impact profitability.
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 heat and high humidity.
• Hide color: No surprise here. 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 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 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.