AHLU (Accumulated Heat Load Unit)

AHLU (Accumulated Heat Load Unit) is a measure of the amount of heat that may be stored in a body.

An animal’s “heat energy balance” is determined by the duration of exposure above the HLI Threshold. Cattle may accumulate heat during the day (the body temperature rises) and dissipate this heat during the night. If there is insufficient night cooling, cattle may enter the following day with an accumulated heat load. This is the AHLU value.

More about the calculation of AHLU

For every hour that the HLI is above the HLI Threshold, heat will be stored in cattle and will progressively increase if the conditions remain the same. The increase is equal to the difference between the HLI and the HLI Threshold.

Heat will only begin to dissipate when the HLI decreases to below the lower threshold. Current research indicates that this threshold is 77 for most breeds, but may be higher for Bos Indicus.

The rate of decrease is determined by the difference between the HLI and the threshold of 77 divided by 2 (i.e. the rate of heat dissipation is half that of heat accumulation). The maximum rate of dissipation is 14.5 units per hour.

The AHLU Risk Level is displayed on the Cattle Heat Risk dashboard as the colours highlighting the AHLU values. This is based on the AHLU, panting scores and risk of a heat event, as shown in the below table:

Worked Example

This worked example will help with better understanding the HLI and AHLU. This uses example data from a demo site, with all values rounded to whole numbers.

Start of the Day

At the start of this day, this site’s HLI is 64, and all AHLUs are 0. This means there should be no accumulated heat from the previous day.

At 7:00 the HLI rises to 83. For AHLU80 cattle, this is above their HLI Threshold (80), so will result in some accumulated heat. The accumulated heat for one hour is the difference between the HLI (83 in this case) and the cattle’s HLI Threshold (80 for AHLU80). Therefore the accumulated heat for AHLU80 cattle increases to 3.

This continues at 8:00, where the HLI rises to 87. This means AHLU80 increases by 7 (to 10 total), AHLU83 increases by 4, and AHLU86 increases by 1.

This AHLU accumulation continues during the day, as the HLI remains above HLI Thresholds.

End of the Day

As the HLI cools down, you can see AHLU values dropping once the HLI drops below 77.

This is shown above, where all AHLUs increase or stay the same until 19:00, where the HLI drops to 66. The decrease in AHLU for one hour is then the difference between the HLI (66 in this case) and 77, divided by 2. This is 11/2 = 5.5. At 19:00, all AHLUs are therefore decreased by 5 or 6 (depending on rounded decimals).

This dissipation continues overnight, however, for AHLU80 cattle, not all heat is dissipated before the next morning. This means the high-risk cattle will start the next day with a heat load.

It should also be noted that the rate of heat dissipation is half that of heat accumulation, so it takes a lot longer for heat to dissipate.