Understand and lessen heat stress when working cattle
Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist
Here they come! Those four little words that Oklahoma cattle producers despise in the summer time: “High Pressure Heat Dome”. Understanding and avoiding heat stress in cattle can be a valuable management tool for summertime in Oklahoma. Most areas of Oklahoma have 10 or more days each year above 100 degrees and 70 or more days with high temperatures above 90 degrees Fahrenheit. (Source: 1997 Oklahoma Climatological Survey). This means that most cow calf operations will be working cattle on days when heat stress to cattle is likely.
Cattle have an upper critical temperature approximately 20 degrees cooler than humans. When humans are a little uncomfortable at 80 degrees and feel hot at 90 degrees, cattle may well be in the danger zone for extreme heat stress. Humidity is an additional stressor that intensifies the heat by making body heat dissipation more difficult. Knowledge of daily body temperature fluctuations and the impact of handling cattle on body temperature can be valuable at this time of year.
Oklahoma State University research with rumen temperature boluses has shown that the core body temperature of beef cows peaks at 2 to 5 hours after the highest daytime temperature (Pye, Boehmer, and Wettemann. 2011 ASAS Midwest Abstracts Page 104; Abstract 285). On a hot spring/summer day the highest daytime temperature is often late afternoon. Therefore the peak body temperature of cattle will occur at 6 PM to 11 PM. In this study, average 4 pm ambient temperature was 93 degrees F., core body temperature was 102.2 F. At 7 pm the ambient temperature had dropped to 88.7 degrees, but core body temperature increased to 103 F. Elevated core body temperatures have been implicated from other research in reduced pregnancy rates in heat stressed cattle.
Research data has been reported by Dr. Mader at the University of Nebraska research station near Concord, Nebraska. He found that moving yearling cattle just a small distance (2000 feet) during mild summer temperatures (80 degrees F.) could change the core body temperature by as much as 1.4 degrees F. This indicates that body temperatures of excited, stressed cattle being worked in hotter temperatures could rise to dangerous levels.
Increases in core body temperatures have been shown to reduce conception rates at the time of artificial insemination and reduced embryo survival and viability for at least 14 days following breeding. If cattle working procedures are still necessary for breeding or for health purposes, during the heat of summer, following a few common sense guidelines may lessen the impact of heat:
1. During hot weather, cattle should be worked before 8:00 am, if possible. Certainly all cattle working must be complete by about 10:00 am. While it may seem to make sense to work cattle after sun down, they may need at least 6 hours of night cooling before enough heat is dissipated to cool down from an extremely hot day.
2. Cattle that must be handled during hot weather should spend less than 30 minutes in the working facility. Drylot pens and corrals loaded with cattle will have very little if any air movement. Cattle will gain heat constantly while they are in these areas. Therefore a time limit of one-half hour in the confined cattle working area should limit the heat gain and therefore the heat stress.
3. Make every effort to see that cool, fresh, water is available to cattle in close confined areas for any length of time. During hot weather conditions cattle will drink more than 1% of their body weight per hour. Producers need to be certain that the water supply lines are capable of keeping up with demand, if working cattle during hot weather.
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