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NACP

Cattle Comfort Index Forecast


About the Heat Load Index (HLI)

The heat load index (HLI) is a model that combines temperature, relative humidity, solar radiation and wind speed into a single index.

The HLI was developed and evaluated in unshaded cattle feedlots in Australia and the United States (Gaughan et al. 2018), with heat thresholds determined from animal respiration rates.

The HLI thresholds are dependent on factors such as cattle breed, coat colour, nutrition, access to water and animal acclimatisation to local conditions.

For Angus cattle, the critical HLI threshold at which the animal will begin to accumulate heat is an HLI of 86. For Waygu, the threshold is 90 to 91 and for Brahman, the threshold is 96. These thresholds are shown on the forecast map colour legend.

The lower threshold at which an animal will dissipate its heat load is 77. If the hourly HLI values are between the low and high thresholds, then an animal's accumulated heat load will remain constant.

Cattle exposed to prolonged heat loads are likely to have a higher core body temperature, reduce their feed intake and impact their fertility rates. Calves and weak cattle are highly susceptible to high heat loads.

About the Cattle Thermal Stress Forecasts

The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G3 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G3 is approximately 12.5 km.

The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.

The ACCESS-G3 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.

Further reading

About the Cattle Comfort Index (CCI)

The Cattle Comfort Index (CCI) is used for monitoring cattle condition during hot and cold weather. The thermal effects of relative humidity (RHadj), wind speed (WSadj), and solar radiation (RADadj) are adjusted to conform to the units of surface mean air temperature (T; °C), where:

CCI = T + RHadj + RADadj + WSadj

The CCI thresholds for cattle are based on young and/or non-acclimatised animals with high susceptibility (for more information, please refer to Mader et al. 2010 and Wang et al. 2018):

CCI stress category Cool/Cold conditions Warm/Hot conditions
No stress > 5°C < 25°C
Mild 5 → 0 °C 25 → 30°C
Moderate 0 → -5°C 30 → 35°C
Severe -5 → -10°C 35 → 40°C
Extreme -10 → -15°C 40 → 45°C
Extreme danger < -15°C > 45°C

As cold stress levels have been developed by researchers using feedlot cattle data and weather observations in North America's Midwest, the thresholds for Australian tropical cattle breeds are likely to be at higher temperatures (i.e., in the 10-15°C range).

About the Cattle Thermal Stress Forecasts

The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G3 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G3 is approximately 12.5 km.

The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.

The ACCESS-G3 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.

Further reading

About the Temperature Humidity Index (THI)

The Temperature Humidity Index (THI) combines air temperature and relative humidity into one index, making it relatively easy to calculate. Although originally developed as an index for measuring human discomfort, the THI became useful in quantifying heat stress in cattle.

THI Calculation

The THI is calculated as:

THI = (1.8 × Ta + 32) - [(0.55 - 0.0055 × RH) × (1.8 × Ta - 26)]

where Ta is air temperature (°C) and RH is the relative humidity (%). This definition of THI was developed for measuring heat stress in cattle in the outdoors (Wang et al. 2018).

THI Thresholds for Cattle

Cattle type Heat stress category THI range
Beef Mild (all breeds) 72 ≤ THI ≤ 77
Significant (Bos taurus) 78 ≤ THI ≤ 86
Significant (all breeds) 87 ≤ THI ≤ 98
Severe (all breeds) THI ≥ 99
Dairy Low-mild 68 ≤ THI ≤ 71
Mild-moderate 72 ≤ THI ≤ 79
Moderate-severe 80 ≤ THI ≤ 89
Severe THI ≥ 90

About the Cattle Thermal Stress Forecasts

The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G3 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G3 is approximately 12.5 km.

The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.

The ACCESS-G3 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.

Further reading


About the Accumulated Heat Load Unit (AHLU)

The continuous buildup of heat in cattle is measured by the Accumulated Heat Load Unit (AHLU). The AHLU is defined as the amount of heat accumulated in cattle over a day or longer, when the Heat Load Index (HLI) exceeds an upper critical threshold. These upper limits vary across cattle breeds and have been determined from feedlot cattle respiration rates (i.e., panting scores; Gaughan et al. 2008). The lower threshold is generally at an HLI = 77. For cattle, if the HLI stays above their upper threshold over the course of a day and night, then their AHLU will increase. When HLI values sit between the lower and upper HLI thresholds, then an animal’s AHLU will remain constant.

For Australia's most common cattle breeds, the upper HLI thresholds are shown below.

Cattle breed Extra information Upper HLI threshold
Angus 100% Bos taurus 86
Hereford, Poll Hereford, Shorthorn (southern adapted), Limousin, Gelbvieh 86
Red Angus 87
Murray Greys 88
Simmental Swiss breed 88
Senepol 89
Limousin x Indicus French breed 89
Shorthorn (northern adapted) 90
Brahman x Angus Not Brangus 90
Charbray 91
Braford 38% Bos indicus 91
Wagyu (Black) 92
Ultrablacks 92
Santa Gertrudis 38% Bos indicus 93
Belmont Red 93
Brangus 38% Bos indicus 93
Senepol x Brahman, Droughtmaster x Brangus 93
Droughtmaster 50% Bos indicus 94
Brahman x Droughtmaster 94
Brahman 100% Bos indicus 96

There are three main AHLU risk classifications:

  • Moderate: 21 ≤ AHLU ≤ 50
  • High: 51 ≤ AHLU ≤ 100
  • Extreme: AHLU >100

As each risk category is exceeded, this increases the likelihood of heat-induced stress within cattle, leading to reduced feed intake, hyperthermia (e.g., high core body temperature), and eventual death.

About the Cattle Thermal Stress Forecasts

The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G3 (Australian Community Climate and Earth-System Simulator – Global version 3). The horizontal resolution of ACCESS-G3 is approximately 12.5 km.

The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.

The ACCESS-G3 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.

Further reading