Subtle variations in the combination of temperature and humidity can mean significant differences in the amount of stress on cattle.
You can find all kinds of indices – the temperature and humidity index being one of the most popular – for measuring weather-induced misery. You can even find some geared specifically toward livestock.
For instance, the Mesonet Cattle Heat Stress Index (CHSI) was developed specifically for grazing cattle (there's a similar index for cold-weather stress). It was developed by Oklahoma State University, in conjunction with the Intermountain Fire Sciences Lab of the U.S. Forest Service based at Missoula, MT.
The CHSI formula is: THI = tair – [0.55-(0.55 x relh/100)] x (tairf-58.8), where THI = Temperature-Humidity Index; tair = air temperature in Fahrenheit; and relh = percent relative humidity.
You can find the formula and other specifics at agweather.mesonet.org/.
According to Mesonet analysts:
• CHSI ≤71 – "Cattle are comfortable and production will not be sacrificed due to severe environmental conditions."
• CHSI 72-79 – Mild Stress – "This is often when cattle will move to shade to cool themselves. Livestock managers should monitor the weather and prepare to take action if conditions worsen.
"Cattle may need more than 2 gals. of water/100 lbs. of body weight. Provide enough tanks for cattle to be able to get the water they need. If possible, water should be cooled. Tanks should be cleaned weekly to encourage water consumption. Making water available under a shaded area will increase cattle water consumption.
• CHSI 80-89 – Moderate Stress – "This is the time to implement management strategies to help reduce cattle stress."
• CHSI ≥ 90 – Severe Stress
Among the management strategies suggested:
• Avoid handling cattle
"If cattle must be worked on days when the CHSI is likely to go over 80, try to do the work before 8 a.m. and keep the maximum time in the holding facilities to no more than 30 minutes. On days when the index will be 80 or above, do not work cattle after 10 a.m. The 60-hour forecast component of the cattle stress index will allow you to schedule management practices to best maintain cattle health."
• Change feeding patterns
"Shift the feeding schedule toward evening on days when the CHSI is above 72. Try to deliver 70% of the daily scheduled feed two to four hours after the peak air temperature. Providing only small amounts of feed during the heat of the day will decrease the metabolic heat of digestion."
• Provide shade
"A shade tree is just as welcome a relief for cattle as humans on a hot summer day. Shade can also be constructed. Shade height should be 8-14 ft. tall and should be large enough to provide 20-40 sq. ft./animal. The most effective shade is a solid reflective roof constructed of white-colored, galvanized or aluminum materials. Shading with wooden slats, plastic fencing or other materials that allow flecks of sunlight to hit the animals are less effective.
"If possible, two shaded areas are recommended, one over the feed area to increase feeding time, and another away from the feed area to encourage the cattle to rest. Water should be made available under both shaded areas, to increase the water consumption during heat stress periods. If the structure is left up year-round, construct a frame adequate for snow load. Shade is insurance against mortality loss. Any performance benefits are a bonus."
• Improve airflow
"Consider where the cattle are located and if there is any air restriction. Buildings, high fences or vegetation can block airflow. A 6-ft.-high windbreak can obstruct airflow for 60 ft. downwind."
• Control biting flies
"Stable flies cause cattle to bunch and disrupt cooling. Monitor the situation and control the flies as needed. Eliminate any shallow pools or muddy areas nearby, since they are common breeding areas for flies."