What is an Accumulated Heat Load Unit (AHLU) and how is it calculated?

The Accumulated Heat Load Unit (or AHLU) is a measure of the amount of heat that may be stored a body. An animal’s ‘heat energy balance’ is determined by the duration of exposure above the Heat Load Index (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 (AHL; Hahn and Mader, 1997).

On CHLT, Accumulated Heat Load Units are calculated every hour, using the HLI for that hour, and the HLI Threshold.

For every hour that the HLI is above the HLI Threshold, heat will be stored in lot cattle and will progressively increase without intervention if the conditions remain the same.

Figure 1: Relationship between HLI and HLI Threshold

Figure 1: Relationship between HLI and HLI Threshold

As illustrated by Figure 1, for every 1 hour period that the HLI (green contour lines) is greater than HLI Threshold (orange line), heat load increases (red areas) at a rate equal to the difference between HLI and the HLI Threshold. (Note: The HLI Threshold can move up and down).

For example, around 10am on Day 1, the HLI Threshold = 86 and the HLI = 98. Therefore the heat load is accumulating at a rate of 12 AHLU/hr.

Figure 2: Relationship between HLI and heat dissipation

Figure 2: Relationship between HLI and heat dissipation

Heat will only begin to dissipate when the HLI decreases to below the lower threshold (shown by blue areas in Figure 2). Current research indicates that this threshold is 77 for most breeds, but may be higher for BosIndicus.

The HLI Threshold is important to understand as it tells you at what point your cattle may start accumulating heat. The HLI threshold will change for even one beast as they go through the feedlot process.

To understand how the HLI Threshold changes use the HLI Threshold calculator (right sidebar of the CHLT Toolbox).

Undertaking activities when the environment is cooler may not necessarily mean the cattle have a lower heat load (see Figure 3).


Figure 3 shows the HLI (green contour line) and the AHLU (red vertical lines) over a three day period. The high HLI on Day 1 and minimal recovery time overnight means that the heat load on Day 2 is actually higher than on Day 1, even though the HLI is lower.

Note: AHLU86 means an AHLU with a HLI Threshold of 86.