Heat stress is defined as any combination of temperature, humidity, radiation and wind producing conditions that are higher than the temperature range of the animal's thermal neutral zone. An animal's performance during heat stress periods depends upon several weather factors, especially temperature and humidity. Thus we can use these factors in our management planning.
The Temperature Humidity Index (THI), a measure of heat stress, combines the effects of temperature and humidity into one value. The chart shows THI for a range of air temperatures and relative humidities.
| Air | Relative Humidity, % | |||||
| Temp | 40 | 50 | 60 | 70 | 80 | 90 |
| 76 | 70 | 71 | 72 | 73 | 74 | 75 |
| 78 | 71 | 73 | 74 | 75 | 76 | 77 |
| 80 | 73 | 74 | 75 | 76 | 78 | 79 |
| 82 | 74 | 75 | 77 | 78 | 79 | 81 |
| 84 | 75 | 77 | 78 | 80 | 81 | 83 |
| 86 | 77 | 78 | 80 | 81 | 83 | 84 |
| 88 | 78 | 80 | 81 | 83 | 85 | 86 |
| 90 | 79 | 81 | 83 | 85 | 86 | 88 |
| 92 | 80 | 82 | 84 | 86 | 87 | 90 |
| 94 | 82 | 84 | 86 | 88 | 90 | 92 |
| 96 | 83 | 86 | 88 | 90 | 92 | |
| 98 | 85 | 87 | 89 | 91 | ||
| 100 | 86 | 88 | 91 | |||
| 102 | 87 | 90 | 92 | |||
| 104 | 88 | 91 |
The Livestock Safety Index (LSI), based on THI, was introduced to alert producers of potential heat stress periods. The LSI contains three stress categories.
Livestock Alert. LSI of 75-78: When the index reaches this range, heat stress will first appear. Precautionary measures should be taken to reduce heat stress conditions in confinement houses.
Livestock Danger. LSI of 79-83: An index in this category is dangerous for confined animals. Disaster can strike in the upper part of this range if managers become lax.
Livestock Emergency. LSI of 84 and higher: These conditions are most likely to form when air temperature exceeds 90 degrees. No cloud cover and little air movement are additional hazards found in such heat stress weather. Livestock should not be worked or shipped when the index reaches this level.
Factors other than temperature and relative humidity can impact heat stress. The addition of sunshine can add several degrees to the THI. Wind can lower the THI by a few points thanks to its bringing cooler air to the animal and carrying away excess heat.
During a typical summer day, the LSI moves from one category to another. It rises from an alert category in the morning to the Emergency category during late afternoon and early evening hours. When air temperatures exceed 86 with 90% relative humidity, emergency conditions will prevail. The same category holds at 40% relative humidity and an air temperature of 97 degrees.
When the air temperature is above 80 degrees, death loss is closely related to THI.
For additional information on animal heat stress, consult your county extension office.
Do not be afraid to let the cows lose a little condition (100-200 lb) that they can put back on easily when feed supplies are plentiful again.