How to calculate THI and read heat stress in cattle

Ask a farmer in April how the herd is doing and you may hear "they're eating less" or "she didn't hold to the AI". Ask what the weather was like and you will hear a temperature. But a thermometer only tells half the story. A hot, dry afternoon and a hot, sticky afternoon feel very different to us, and they feel different to cattle too.

The temperature-humidity index (THI) combines the two numbers into one. This post shows the formula, works through it step by step for a hot afternoon, and then looks at what Thai studies found: how often Thai weather crosses the stress lines, and how much pregnancy rates dropped in a Thai beef herd in the hotter months.

It is an advanced post because there is some arithmetic. You only need a calculator, and every research figure below links to the paper it came from.

Why humidity matters as much as heat

An extension guide from the University of Nebraska–Lincoln puts it plainly: heat stress is hard on livestock, especially when it comes with high humidity and low wind speed. The same guide says environmental stress depends on four things: temperature, humidity, wind speed and solar radiation (Chichester & Mader, UNL Extension G2121).

THI uses only the first two. It leaves out wind and sun, so it is a simplification, but the two numbers it needs are easy to get every day. A Kasetsart University team that studied Thai dairy herds describes THI as an indicator of the stress caused by high air temperature and humidity, used to decide whether animals are in a comfort zone or a stress zone (Sae-tiao et al. 2019, LRRD 31(12)).

The formula

The Kasetsart study prints the formula it used, which comes from a 1976 NOAA publication on livestock hot weather stress (Sae-tiao et al. 2019):

THI = (1.8 × T + 32) − (0.55 − 0.0055 × RH) × (1.8 × T − 26)

T is the average dry-bulb air temperature in °C, and RH is the relative humidity in %.

It looks heavy, but each part has a job:

• 1.8 × T + 32 is simply the temperature converted to Fahrenheit. The index starts from "how hot is it".
• (0.55 − 0.0055 × RH) is a humidity discount. When the air is dry, this number is bigger, so more is taken away. As RH rises, the discount shrinks. At 100% RH it reaches zero, and THI equals the Fahrenheit temperature.
• (1.8 × T − 26) scales that discount by the heat. Humidity matters more on a hot day than on a cool one.

So THI is "the temperature, minus a bit for dry air". Humid air takes away the discount.

A worked example: a hot afternoon

Let's take an afternoon with 35 °C and 60% RH. These are example numbers, not a measurement from any one place.

Now change one thing. Keep 35 °C but make the air drier, at 40% RH. Step 3 becomes 0.55 − 0.22 = 0.33, step 5 becomes 0.33 × 37 = 12.21, and THI drops to about 82.8. The thermometer reads the same, but the humid afternoon scores about four points higher.

And a cool, humid morning at 26 °C and 85% RH gives 78.8 − (0.0825 × 20.8) ≈ 77.1. Mornings are not automatically "safe" in a humid climate. They are just less bad.

THI = (1.8T+32) - (0.55-0.0055RH)(1.8T-26)                      
                                                        *       
              *                                  *              
          *                              **                     
TIME 04:00                                                      
                                                                
  [·]      [·]                                                  
  [·]      [▒]                 MILD       +          MODERATE   
  [·]      [█]                          ·········               
  [·]•───────────•────────────┐     ····         ····           
  [·]      [█]                ├>+ ···               ···         
  [·]      [█]•───────•───────┘  ··                   ··        
  [░]      [█]                  ·· \\\                 ··+      
  [█]      [█]                  ·     \\\\              ·       
 TEMP      RH               OK  ·         \\o           ·       
 22.2C    89%                 68                        90      
                                        THI 71.2                
__________________________                 OK                   

Check yourself. Different papers use different THI formulas. A 2024 Thai dairy study, led from Kasetsart University's veterinary faculty, uses THI = 0.8T + (RH/100) × (T − 14.4) + 46.4 (Thammahakin et al. 2024, Animals). For 35 °C and 60% RH that gives about 86.8, close to the NOAA version here, but always compare a THI with the thresholds from the same paper.

Where the stress lines are

The Kasetsart dairy study used these bands (Sae-tiao et al. 2019):

The paper's own table draws the moderate band slightly differently (77 up to 88), so treat the edges as approximate. It also notes, citing earlier work, that dairy cows are likely to begin experiencing heat stress when THI exceeds 72.

These lines were set for dairy cows. Other studies draw them elsewhere. The 2024 study above, with its own formula, calls 72 to 79 mild, 80 to 89 moderate and above 90 severe (Thammahakin et al. 2024). A climate-risk study of Northeast Thailand assumed that production losses become likely at a THI of 84 or more (Somparn et al. 2004, International Journal of Biometeorology).

How often Thai weather crosses those lines

The Kasetsart team computed THI from 85,824 daily records at 17 Thai Meteorological Department stations in five regions, from 2002 to 2015 (Sae-tiao et al. 2019). Using the bands above:

• Over the whole period, 67.4% of days were in the moderate band, 24.3% mild and only 8.3% in the comfort zone.
• By region, the share of moderate-stress days ran from 52.2% in the Northeast to 81.2% in the East.
• By month, moderate stress ranged from 12.7% of days in January to 95.2% in June. The comfort zone was 0% in June, July and August.
• The paper's seasons are winter (November to February), summer (March to June) and rainy (July to October). Summer was 87.1% moderate, the rainy season 85.7%. Winter was the only season with a real share of comfortable days (23.7%).

The results also show that the average THI in every region was above 72.

The older Northeast study looked at extremes instead. Using three-hourly records from 15 stations between 1990 and 1999, it reports that, across the study area, the mean total number of days with THI at or above 84 was 56 (the abstract does not say over what period this total is counted). It also found a strong north-to-south gradient and the highest risk in the southern part of the region (Somparn et al. 2004).

How long the hot season lasts

The 2024 study in Animals sums up earlier Thai work this way: on average, a high THI is present for eight months of the year, March to October, for about 9 hours a day, from 10 a.m. to 7 p.m. It cites an analysis of THI in Nakhon Pathom province from 2008 and 2009 for that statement (Thammahakin et al. 2024). That is one province, so read it as a guide rather than a national rule. It fits the station data above, though: the cool months are the exception.

What heat did to pregnancy rates in Thai beef cattle

The clearest Thai beef study I found is by Kajaysri and Wattananorasate, published in the Thai Journal of Veterinary Medicine in 2018 (Kajaysri & Wattananorasate 2018). The setup:

• Animals: 81 Brahman heifers and 95 postpartum beef cows on one commercial farm in Chonburi province, studied from November 2014 to October 2015.
• Protocol: Ovsynch plus a CIDR for 7 days, timed AI on day 10, pregnancy checked about 60 days later.
• Weather: seasonal averages of the daily temperature and humidity records kept by the Meteorological Department in Chonburi province. Winter (27.9 °C, 65.75% RH) was classed as mild stress at a THI of about 77. Summer (30.55 °C, 71% RH) and the rainy season (29.78 °C, 73.25% RH) were classed as moderate stress, at about 82 and 81.

The paper doesn't print its THI formula. As a cross-check, I put its seasonal averages into the NOAA formula above and got about 77.7, 82.4 and 81.5, close to the values the authors report.

The combined difference was statistically significant (P < 0.05). Heifers and cows on their own each showed the same direction but only as a trend (P = 0.10 and P = 0.09). Heifers and cows did not differ from each other in any season.

The authors explain the drop using earlier research: stress hormones can suppress the signals that drive ovulation, sperm may survive less well in an overheated cow, and heat can harm the early embryo. Their practical advice is to run this synchronisation and timed-AI programme in the cool winter months to get the best pregnancy rates.

Two points stand out to me. First, the heifers were Brahman, a breed type known for heat tolerance, and their pregnancy rate still fell, although for heifers on their own the difference was only a trend. Second, the "moderate" months were most of the year.

A dairy study from western Thailand points the same way: under moderate heat stress, first-service conception was 15% (3 of 20) against 42.11% (8 of 19) under mild stress. That was a trend, not a significant result (p = 0.06), and those were crossbred Holstein dairy cows, not beef cattle (Thammahakin et al. 2024).

Signs of heat stress to watch for

From the sources in this post:

• Behaviour: bunching together, seeking shade, panting, slobbering or heavy salivation, foam around the mouth, open-mouth breathing, poor coordination and trembling. The Nebraska guide says that if you see these, assume the animal is carrying a heat load (UNL G2121).
• Body: cows under moderate heat stress had a higher rectal temperature (38.87 against 38.47 °C) and a faster breathing rate (51.17 against 41.04 breaths per minute) than cows under mild stress (Thammahakin et al. 2024).
• Breeding: heat stress shortens heat and makes it weaker, so heat detection becomes less reliable (Kajaysri & Wattananorasate 2018). In the dairy study, "silent heat" (a cow that showed no heat behaviour even though a hormone test or a new corpus luteum showed she had cycled) was seen in 40.91% of cows under moderate stress (9 of 22) against 13.64% under mild stress (3 of 22) (Thammahakin et al. 2024).

What you can do about it

None of this is new advice, but now it has a number behind it. The general good practice from the sources:

1. Shade. Trees, buildings or sunshades. The Nebraska guide suggests sunshades at least 10 feet (about 3 metres) off the ground so air can move under them (UNL G2121). The Thai papers suggest shades and fans (Kajaysri & Wattananorasate 2018; Sae-tiao et al. 2019).
2. Plenty of clean, cool water. At 90 °F (about 32 °C), cattle may drink almost twice as much as at 70 °F (about 21 °C). Add extra troughs if space at the water is tight (UNL G2121).
3. Work cattle in the morning. Body temperature tends to peak in the early evening and is lowest just before sunrise. Processing cattle can raise it by ½ to 3½ degrees (the guide does not state the unit), so avoid late-afternoon and early-evening work. Move animals in the evening or early morning if you must transport them (UNL G2121). Penning cows for AI is handling too.
4. Plan synchronisation for the cool months where you can, as the Chonburi study recommends (Kajaysri & Wattananorasate 2018).
5. Wetting, done right. Use large water droplets. A fine mist can just add humidity, which pushes THI up (UNL G2121).

07:00 SUN LOW                                                   
                                                                
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                                    &&&*&'''*&**'*'''           
                                           ||                   
             _^______          _^______ \/ ||                   
           \/        \       ~/        \(oo)|                   
          '(          )    ' (          )  ||                   
|~~~----|   \________/\/      \________/   ||                   
\_______/    ||    || (oo)     ||    ||  ░/||\░░░░░░░░░░░░░░░░░░
_________________________________________▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒

Why native and Brahman-type cattle cope better

A review of Thai beef production describes Thai native cattle as zebu-type (Bos indicus), with the typical hump, and says they are well adapted to the tropics, with good heat tolerance and resistance to external parasites. The same review says the Kabinburi breed, half German Simmental and half Brahman, was developed for better heat tolerance, fertility and growth, and it mentions the heat tolerance of Brangus (Bunmee et al. 2018, Asian-Australasian Journal of Animal Sciences).

Why? A review by Hansen in Animal Reproduction Science (2004) explains that zebu cattle control body temperature better under heat stress because they have a lower metabolic rate (they make less heat) and a greater capacity to lose heat. Compared with European breeds, their tissues move heat from the body core to the skin more easily, their sweat glands are larger, and their hair coat helps heat loss and absorbs less sunlight. Even their early embryos are less likely than European-breed embryos to stop developing at high temperature (Hansen 2004, abstract via FAO AGRIS).

That's an advantage, not immunity. In the Chonburi study above, the Brahman heifers' pregnancy rate fell from 72.41% under mild stress to 53.85% under moderate stress. That was a trend rather than a significant result (P = 0.10), but it points the same way as the whole herd.

A simple habit

If you want to use THI on your farm, you don't need a weather station. Write down the temperature and humidity at the same times each day, for example early morning and mid-afternoon. Work out THI with the formula, and keep it next to your breeding records. After a season, you can line up AI dates against THI and see whether your own herd follows the pattern in the studies. My guess is it will, but the notebook will tell you for sure.

Sources

• Sae-tiao T., Laodim T., Koonawootrittriron S., Suwanasopee T., Elzo M.A. (2019). Tropical climate change and its effect on milk production of dairy cattle in Thailand. Livestock Research for Rural Development 31(12).
• Kajaysri J., Wattananorasate A. (2018). Influence of heat stress on pregnancy rate of beef cattle heifers and cows using Ovsynch plus CIDR followed by timed artificial insemination in Thailand. Thai Journal of Veterinary Medicine 48(1): 71–77.
• Somparn P., Gibb M.J., Markvichitr K., Chaiyabutr N., Thummabood S., Vajrabukka C. (2004). Analysis of climatic risk for cattle and buffalo production in northeast Thailand. International Journal of Biometeorology 49(1): 59–64. doi:10.1007/s00484-004-0206-6
• Thammahakin P., Yawongsa A., Rukkwamsuk T. (2024). Effect of Heat Stress on Subsequent Estrous Cycles Induced by PGF2α in Cross-Bred Holstein Dairy Cows. Animals 14(13): 2009. (publisher page)
• Bunmee T., Chaiwang N., Kaewkot C., Jaturasitha S. (2018). Current situation and future prospects for beef production in Thailand: A review. Asian-Australasian Journal of Animal Sciences 31(7): 968–975.
• Hansen P.J. (2004). Physiological and cellular adaptations of zebu cattle to thermal stress. Animal Reproduction Science 82: 349–360 (record and abstract in FAO AGRIS).
• Chichester L.M., Mader T.L. Heat Stress: What You Should Know to Make Livestock Shows a Success, University of Nebraska–Lincoln Extension, G2121 (2012).
• Photo: Most, but not all, cattle in a field in Ellis County, Texas, grab some shade on a sunny afternoon, Carol M. Highsmith, public domain (Library of Congress), Wikimedia Commons
• Photo: Drought Mitigation - Solar Powered Cattle Watering, USDA NRCS, public domain, Wikimedia Commons
• Photo: Oxen in rice farm Thailand, Love Krittaya, public domain, Wikimedia Commons