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.
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. The increase is equal to the difference between the HLI and the HLI Threshold.
The 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.