Forecasting Heat Stress
Creating Heat Stress Forecast Maps
The heat stress forecast maps are made using the seven day forecasts of four weather parameters (temperature, humidity, wind speed, and cloud cover) from the National Oceanic and Atmospheric Administration (NOAA) - National Weather Service (NWS). This website accesses the forecast information to produce a map showing stress categories by color. The prediction of animal stress is based on an equation that combines weather forecast data to estimate cattle heat stress response. These maps are to be used as an estimate of general trends of stress levels over the forecast period. The maps are also designed to forecast the anticipated peak heat stress category for each day. The maps cannot predict individual animal stress levels since there is much variability among animals.
Important Weather Parameters
The estimates of heat stress are based on four specific weather parameters: temperature, wind speed, humidity, and solar radiation. Each parameter plays a significant role in the overall heat balance of feedlot cattle.
In cool conditions it is easy for the animal to lose heat. As temperature increases it becomes increasingly difficult to lose heat.
Wind or air movement over the animals helps the animals lose heat. Even a gentle breeze can provide relief on a hot sticky day.
As humidity increases it becomes more difficult to evaporate the water both from the skin surface and from the surface of the respiratory track. Hot, sticky conditions that sometimes follow a summer rain shower can make respiratory heat loss very difficult.
Solar radiation is the energy the sun transmits. Radiant solar energy heats all objects that it impacts. Darker objects absorb more solar radiation than lighter objects. Forecast estimates of solar radiation are based on cloud cover, which is a forecast parameter provided by the National Weather Service.
Breathing Rate Equation
Breathing rate varies as cattle attempt to maintain thermal balance. Therefore, breathing rate reacts very quickly to changes in the weather. Studies have shown breathing rate to be a good indicator of cattle heat stress and is easily observable without special equipment or the need to handle the animal.
Using breathing rate as the indicator, an equation was developed by researchers at the US Meat Animal Research Center to predict the animal?s response to different weather conditions. The equation has been validated by subsequent studies conducted during summer conditions. The environmental parameters used to predict the animal's response include temperature, humidity, wind speed, and solar radiation.
For temperatures higher than 80?F
Breathing Rate = (2.83 x Temperature) + (0.58 x Humidity) - (0.76 x Wind Speed) + (0.039 x Solar Radiation) - 196.4
The chart below illustrates stress categories as defined by predicted breathing rate.
|Predicted Breathing Rate||Heat Stress Category|
|Less than 90 breaths per minute||Normal|
|From 90 - 110 breaths per minute||Alert|
|From 110 - 130 breaths per minute||Danger|
|Above 130 breaths per minute||Emergency|
In the breathing rate equation above, the breathing rate is the predicted breathing rate of cattle in a feedlot in breaths per minute. Temperature is air temperature in degrees Fahrenheit. Humidity is relative humidity in percent. For example, 60% relative humidity would be used as 60. Wind speed is independent of direction and measured in miles per hour. Solar radiation is the energy from the sun in Watts per meter2. If access to on-site weather data is not available, a good number to use is about 1000 Watts/meter2 which is the maximum solar radiation per day during the summertime. Solar radiation changes during the course of the day and with season and location. A graph of solar radiation is shown below. These measurements were taken at Clay Center, NE on July 5, 2001.
An animal must establish a heat balance to maintain a constant body temperature. Eating and digesting feed, standing, moving or other uses of their muscles all create heat that must be lost to maintain a heat balance. The animal has multiple means of losing heat which include: the simple transfer of heat by conduction (transferring heat to a solid), convection (transferring heat to water or air), and radiation (transferring heat in the form of radiant energy), as well as the evaporation of moisture. Cattle evaporate water not only from the surface of their skin, but also from the surface of their respiratory tract (lungs, trachea, etc.). Water is evaporated from the surface of the respiratory tract during each breath. In order to increase the loss of moisture cattle will pant, therefore breathing rate is an important behavior to watch in cattle during hot weather. Body temperature is another important measure because it is an indicator of how the animal is maintaining their heat balance. Body temperature is difficult to measure without expensive equipment or handling each animal.
Brown-Brandl, T. M., Eigenberg, R. A., Nienaber, J. A., and Hahn, G. L. 2005. Dynamic response indicators of heat stress in shaded and non-shaded feedlot cattle, Part 1: Analyses of indicators. Biosystems Engineering 90(4): 451-462. View Paper
Eigenberg, R. A., Brown-Brandl, T. M., Nienaber, J. A., and Hahn, G. L. 2005. Dynamic Response Indicators of Heat Stress in Shaded and Non-shaded Feedlot Cattle, Part 2: Predictive Relationships. Biosystems Engineering 91(1): 111-118. View Paper